mxs: prevent lockup in endless loops waiting for HW bits to clear

mxs: prevent lockup in endless loops waiting for HW bits to clear

The RTC unit of the i.MX28 can lockup when e.g. the
RTC_PERSISTENT0_CLOCKSOURCE bit is asserted but the XTAL32KHZ_PWRUP is
deasserted. In this case the original code would just lockup waiting
indefinitely for the STALE_REGS or NEW_REGS masks to clear.

The lockup can only be resolved by a real power-on-reset. Advising the
user to do a manual reset is the only thing we can do at this point.