Since commit:

    2ccd451 ARM i.MX28: change default watchdog reset method

the external reset via the reset pin is broken. That commit overwrites the
HW_CLKCTRL_RESET register with only WDOG_POR_DISABLE set, which results in
disabling the external reset.

This patch uses read-modify-write to set the WDOG_POR_DISABLE, leaving the
WDOG_POR_DISABLE untouched.

Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>