mips: remove redundant macro mc146818_decode_year

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 on mach-jazz it is unused and can be removed. On other platforms it
is defined in the same way, so it can be safely folded into
mc146818_get_cmos_time().

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