ARM i.MX28: change default watchdog reset method

The default setting for the imx28 watchdog is to do a power-off reset. If the
SoC is only powered via battery, then the watchdog powers the chip down, though.
According to the datasheet it should still be possible to execute a proper POR
with battery power, but testing showed otherwise.
When the watchdog power-off reset is disabled, a software reset is executed
instead. This works with and without battery power.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Add warning above get_ram_size

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>