fix(smccc): ensure that mpidr passed through SMC is valid

There are various SMC calls which pass mpidr as an argument which is
currently tested at random places in SMC call path.
To make the mpidr validation check consistent across SMC calls, do
this check as part of SMC argument validation.

This patch introduce a helper function is_valid_mpidr() to validate
mpidr and call it as part of validating SMC arguments at starting of
SMC handlers (which expect mpidr as an argument).

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I11ea50e22caf17896cf4b2059b87029b2ba136b1