Add migration guides for GCM

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

docs/3.0-migration-guide.d/gcm-alt.md

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+GCM interface changes: impact for alternative implementations
+-------------------------------------------------------------
+
+The GCM multipart interface has changed as described in [“GCM multipart interface: application changes”](#gcm-multipart-interface:-application-changes). The consequences for an alternative implementation of GCM (`MBEDTLS_GCM_ALT`) are as follows:
+
+* `mbedtls_gcm_starts()` now only sets the mode and the nonce (IV). The new function `mbedtls_gcm_update_ad()` receives the associated data. It may be called multiple times.
+* `mbedtls_gcm_update()` now allows arbitrary-length inputs, takes an extra parameter to indicate the actual output length. Alternative implementations may choose between two modes:
+    * Always return the partial output immediately, even if it does not consist of a whole number of blocks.
+    * Buffer the data for the last partial block, to be returned in the next call to `mbedtls_gcm_update()` or `mbedtls_gcm_finish()`.
+* `mbedtls_gcm_finish()` now takes an extra output buffer for the last partial block if needed.

docs/3.0-migration-guide.d/gcm-multipart.md

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+GCM multipart interface: application changes
+--------------------------------------------
+
+The GCM module now supports arbitrary chunked input in the multipart interface.
+
+For applications using GCM for multipart operations, this means the following changes:
+
+* `mbedtls_gcm_starts()` now only sets the mode and the nonce (IV). Call the new function `mbedtls_gcm_update_ad()` to pass the associated data.
+* The current implementation has a limitation that `mbedtls_gcm_update_ad()` may only be called once. This limitation will be lifted shortly; watch https://github.com/ARMmbed/mbedtls/issues/4351 for updates.
+* `mbedtls_gcm_update()` now takes an extra parameter to indicate the actual output length. In Mbed TLS 2.x, applications had to pass inputs consisting of whole 16-byte blocks except for the last block (this limitation has been lifted). In this case:
+    * As long as the input remains block-aligned, the output length is exactly the input length, as before.
+    * If the length of the last input is not a multiple of 16, alternative implementations may return the last partial block in the call to `mbedtls_gcm_finish()` instead of returning it in the last call to `mbedtls_gcm_update()`.
+* `mbedtls_gcm_finish()` now takes an extra output buffer for the last partial block. This is needed for alternative implementations that can only process a whole block at a time.
+
+Applications using one-shot GCM or using GCM via the `mbedtls_cipher_xxx` or `psa_aead_xxx` interfaces do not require any changes.