SPM: Fix calculation of max page granularity

SPM: Fix calculation of max page granularity

The code was incorrectly reading from ID_AA64PRF0_EL1 instead of
ID_AA64MMFR0_EL1 causing the supported granularity sizes returned by the
code to be wrong.

This wasn't causing any problem because it's just used to check the
alignment of the base of the buffer shared between Non-secure and Secure
worlds, and it was aligned to more than 64 KiB, which is the maximum
granularity supported by the architecture.

Change-Id: Icc0d949d9521cc0ef13afb753825c475ea62d462
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>