MIPS: import gt64120.h header from Linux

The Linux specific register access macros, the
extern function declarations and the UL suffixes
has been removed.

The header file will be used for the qemu-malta
board.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>