tpm: Enable TCG_TPM2_HMAC by default only for X86_64

Given the not fully root caused performance issues on non-x86 platforms,
enable the feature by default only for x86-64. That is the platform it
brings the most value and has gone most of the QA. Can be reconsidered
later and can be obviously opt-in enabled too on any arch.

Link: https://lore.kernel.org/linux-integrity/bf67346ef623ff3c452c4f968b7d900911e250c3.camel@gmail.com/#t


Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>