mtd: rawnand: orion: Handle cases where __LINUX_ARM_ARCH__ is not defined

Make sure __LINUX_ARM_ARCH__ is defined before testing its value.
This is needed if we want to allow selection of this driver when
COMPILE_TEST=y.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>