If we experience a watchdog reset, the indicating flag should persist
till the Linux driver had a chance to see it.

The flag bit is special however in that writing 1 clears the bit and
writing 0 keeps it intact, i.e. :

Bit
read written  result
  0     0     = 0
  1     0     = 1
  0     1     = 0
  1     1     = 0

So in the bootloader, we should write a zero always. The OS on the
other hand can either write 1 or the old value after reading to
clear the flag.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>