mips/mach-rm: remove custom mc146818rtc.h file

The mc146818_decode_year macro is used only in mc146818_get_cmos_time,
which in turn is called only in
	arch/mips/loongson2ef/common/time.c
and
	arch/mips/mti-malta/malta-time.c

So no SNI or mach-rm content. SNI did not use mc146818_get_cmos_time
apparently since
commit 06cf5583 ("[MIPS] SNI RM updates")
in 2007.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>