MIPS: Kconfig: add MIPS_GENERIC_KERNEL symbol

The MIPS_GENERIC symbol now won't select any other configuration option.
The MIPS_GENERIC_KERNEL will select all the options that the previous
MIPS_GENERIC option did select, and will select MIPS_GENERIC as well.

The whole point of this, is that it now becomes possible to compile a
kernel for a SoC supported by the arch/mips/generic/ code, without
making that kernel generic itself.

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