mtd: rawnand: ingenic: Add dependency on MIPS || COMPILE_TEST

This driver has no arch-specific instructions but is only ever useful
on MIPS; so disable this driver if we're not compiling for MIPS, unless
the driver is compile-tested.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200302184509.10666-1-paul@crapouillou.net