The function arm_validate_ns_entrypoint() validates a given non-secure
physical address. This function however specifically returns PSCI error
codes.

Non-secure physical address validation is potentially useful across ARM
platforms, even for non-PSCI use cases. Therefore make this function
common by returning 0 for success or -1 otherwise.

Having made the function common, make arm_validate_psci_entrypoint() a
wrapper around arm_validate_ns_entrypoint() which only translates return
value into PSCI error codes. This wrapper is now used where
arm_validate_ns_entrypoint() was currently used for PSCI entry point
validation.

Change-Id: Ic781fc3105d6d199fd8f53f01aba5baea0ebc310
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>