fix(plat/common): register PLAT_SP_PRI only if not already registered

Build failure is seen in TF-A when RAS and SPM are enabled together.
With recent changes in TF-A to strictly scrutinize the compiler warnings
this issue surfaced.

PLAT_SP_PRI macro is defined to be equal to PLAT_RAS_PRI. When RAS and
SPM both are enabled both try to register with same priority value which
can be avoided. This change ensures that SPM registers its priority only
if the value is different from RAS priority value.

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: Ie14f82d27c9835b24890cc4561a56821881cf0ec