tpm_crb: ffa_tpm: fix/update comments describing the CRB over FFA ABI

-Fix the comment describing the 'start' function, which was a cut/paste
 mistake for a different function.

-The comment for DIRECT_REQ and DIRECT_RESP only mentioned AArch32
 and listed 32-bit function IDs.  Update to include 64-bit.

Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>