video: mmp: Remove references to CPU_PXA988

References to the Kconfig symbol CPU_PXA988 were added to the tree in
v3.9. But that Kconfig symbol has never been part of the tree. So get
rid of these references.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>