drivers: firmware: psci: drop duplicate const from psci_of_match

This is to fix below sparse warning:
drivers/firmware/psci.c:mmm:nn: warning: duplicate const

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>