Remove current CPU mpidr from PSCI common code

Many of the interfaces internal to PSCI pass the current CPU
MPIDR_EL1 value from function to function. This is not required,
and with inline access to the system registers is less efficient
than requiring the code to read that register whenever required.

This patch remove the mpidr parameter from the affected interfaces.

Change-Id: I16120a7c6944de37232016d7e109976540775602