crypto: powerpc/p10-aes-gcm - Re-write AES/GCM stitched implementation

Re-write AES/GCM assembly codes with smaller footprints and
small performance gain.  Handling the partial blocks differently that
computes partial block to AES states and re-assembles to a complete
block and then computes a full-block hash.

Added gcm_update() to update the last partial block hash value and
generate the final digest.

Fixes: fd0e9b3e ("crypto: p10-aes-gcm - An accelerated AES/GCM stitched implementation")

Signed-off-by: Danny Tsen <dtsen@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>