[MIPS] Convert to RTC-class ds1742 driver

The generic rtc-ds1742 driver can be used for RBTX4927 and JMR3927
(with __swizzle_addr trick).  This patch also removes MIPS local
DS1742 stuff.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>