Only define arm_get_rotpk_info_regs if ROTPK is in registers,
i.e. (ARM_ROTPK_LOCATION_ID == ARM_ROTPK_REGS_ID). This will
allow platform build without definition of TZ_PUB_KEY_HASH_BASE
if dedicated registers for ROTPK are not available on the platform.

Change-Id: I74ee2d5007f5d876a031a1efca20ebee2dede0c7
Signed-off-by: Usama Arif <usama.arif@arm.com>