MIPS: ingenic: Default to a generic board

Having a generic board option makes it possible to create a kernel that
will run on various Ingenic SoCs, as long as the right devicetree is
provided.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>