rtc: fix deadlock

if get_rtc_time() is _ever_ called with IRQs off, we deadlock badly
in it, waiting for jiffies to increment.

So make the code more robust by doing an explicit mdelay(20).

This solves a very hard to reproduce/debug hard lockup reported
by Mikael Pettersson.

Reported-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>