drivers: firmware: psci: make two helper functions static

psci_power_state_loses_context() and psci_power_state_is_valid are only
used internally now, so make them static.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>