Verify capabilities before handling PSCI calls

Verify capabilities before handling PSCI calls

This patch implements conditional checks in psci_smc_handler() to verify
that the psci function invoked by the caller is supported by the platform
or SPD implementation. The level of support is saved in the 'psci_caps'
variable. This check allows the PSCI implementation to return an error
early.

As a result of the above verification, the checks performed within the psci
handlers for the pm hooks are now removed and replaced with assertions.

Change-Id: I9b5b646a01d8566dc28c4d77dd3aa54e9bf3981a