MIPS: jz4740: Drop all obsolete files

Support for Ingenic SoCs is now provided by the arch/mips/generic/ code,
so all files in the arch/mips/jz4740/ folder can dropped, except for the
Kconfig, and the cpu-feature-overrides.h header file.

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