MFC r336070, r336072-r336073, r336076

r336070:
Add pnp info and a module makefile for the imx_wdog watchdog driver.

r336072:
Correctly calculate the value to put in the imx wdog countdown register.

The correct value is seconds*2-1. The code was using just seconds*2, which
led to being off by a half-second -- usually not a big deal, except when the
value was the max (128) it overflowed so zero would get written to the
countdown register, which equates to a timeout of a half second.

r336073:
Add support to the imx watchdog for the FDT "timeout-sec" property, by
automatically initializing the watchdog using the given value. Also,
attach at BUS_PASS_TIMER to extend watchdog protection to more of the
kernel init process.

r336076:
Add a manpage for the imx5/6 watchdog driver.