PSCI: Do psci_setup() as part of std_svc_setup()

This patch moves the invocation of `psci_setup()` from BL31 and SP_MIN
into `std_svc_setup()` as part of ARM Standard Service initialization.
This allows us to consolidate ARM Standard Service initializations which
will be added to in the future. A new function `get_arm_std_svc_args()`
is introduced to get arguments corresponding to each standard service.
This function must be implemented by the EL3 Runtime Firmware and both
SP_MIN and BL31 implement it.

Change-Id: I38e1b644f797fa4089b20574bd4a10f0419de184