powerpc/pseries: Add FW_FEATURE_PLPKS feature flag

Add a firmware feature flag, FW_FEATURE_PLPKS, to indicate availability of
Platform KeyStore related hcalls.

Check this flag in plpks_is_available() and pseries_plpks_init() before
trying to make an hcall.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230224041012.772648-1-ajd@linux.ibm.com