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 06cf558 ("[MIPS] SNI RM updates")
in 2007.

Signed-off-by: Mateusz Jończyk <[email protected]>
Signed-off-by: Thomas Bogendoerfer <[email protected]>

Author: matjon