tpm: declare tpm2_get_pcr_allocation() as static

There's no need to export tpm2_get_pcr_alloation() because it is only
a helper function for tpm2_auto_startup(). For the same reason it does
not make much sense to maintain documentation for it.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>