ARM: mxs: rename function that sets AUTO_RESTART flag

The AUTO_RESTART flag of HW_RTC_PERSISTENT0 register will
power up the chip automatically 180ms after power down.
This bit must be enabled by the boot loader to ensure the
target can start upon hardware reset or watchdog reset
even when powered from a battery.

Currently the function named 'mxs_power_clear_auto_restart()'
is setting this flag although the 'clear' in its name suggest
the opposite.

This patch renames the function to 'mxs_power_set_auto_restart()'
and removes the comment about EVK revision A which was confusing
because the function indeed was setting the flag.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>