MIPS: generic: Add support for Ingenic SoCs

Add support for Ingenic SoCs in arch/mips/generic/.

The Kconfig changes are here to ensure that it is possible to compile
either a generic kernel that supports Ingenic SoCs, or a Ingenic-only
kernel, both using the same code base, to avoid duplicated code.

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