MX31: add support for MX31 watchdog

MX31: add support for MX31 watchdog

The patch add CONFIG_HW_WATCHDOG to be used
with the internal watchdog timer of the MX31
processor. Two function are exported for the
board maintainers:
	mxc_hw_watchdog_enable
	mxc_hw_watchdog_reset

The board maintainer can decide to use mxc_hw_watchdog_reset as
hw_watchdog_reset, or to implement his own function to reset
the watchdog.
The watchdog timer can be configured with CONFIG_SYS_WD_TIMER_SECS
(value in seconds). The MX31 allows values between 0.5
(CONFIG_SYS_WD_TIMER_SECS = 0) and 128 seconds.

Signed-off-by: Stefano Babic <sbabic@denx.de>