Remove mips_rtc_lock

The mips_rtc_lock is no longer needed because RTC operations should be
protected already by other mechanism. (rtc_lock, local_irq_save, etc.)

Also, locking whole rtc_get_time/rtc_set_time should be avoided while
some RTC routines might take very long time (a few seconds).

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