feat(spmd): introduce FFA_PARTITION_INFO_GET_REGS

feat(spmd): introduce FFA_PARTITION_INFO_GET_REGS

Add code in SPMD to forward calls to FFA_PARTITION_INFO_GET_REGS. This
is a new ABI that allows getting partition information without the need
for rx/tx buffer, that helps in situations where having an rx/tx buffer
mapped and available is difficult (ex. uefi runtime services).
Currently, the spmc at el3 does not support this new ABI.
The new ABI uses registers x8-x17 to return partition information so
changes are made to ensure those registers are passed through to the
SPMC and restored on the return path.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I1fe5956763e054e4f8d62292fc1247e7120bb5a4