MIPS: ingenic: Use enum instead of macros for Ingenic SoCs

Use an enum instead of macros to represent the various versions of the
Ingenic SoCs, and add some of the SoC versions that were previously
missing.

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