drivers: firmware: psci: define more generic PSCI_FN_NATIVE macro

This patch replaces the definition and usage of PSCI_0_2_FN_NATIVE with
the new and more generic macro PSCI_FN_NATIVE that can be used with any
version. This will be useful for the new features introduced in PSCIv1.0
and for any future revisions.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>