powerpc/powernv/flash: Check OPAL flash calls exist before using

Currently only FSP based powernv systems supports firmware update
interfaces. Hence check that the token OPAL_FLASH_VALIDATE exists
before initalising the flash driver.

Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210914101630.30613-1-hegdevasant@linux.vnet.ibm.com