MFC r328345, r328349, r328405, r328407, r328442

r328345:
Reformat indentation to match other imx5/6 register definition headers, and
tweak some comments. No functional changes.

r328349:
Make the trivial imx_soc_family() function an inline in imx_machdep.h.

The imx_machdep.c file is on the fast path to non-existance and this would
be the only thing left in it after some watchdog changes are completed.

r328405:
Minor cleanups... Move DRIVER_MODULE() and other boilerplate stuff to the
bottom of the file, where it is in most imx5/6 drivers. Switch from an RD2
macro using bus_space_read_2() to an inline function using bus_read_2();
likewise for WR2. Use RESOURCE_SPEC_END to end the resource_spec list.

Net effect should be no functional changes.

r328407:
Fix return style in RD2. Remove bogus return value from a void function
in WR2 (I have no idea why that didn't result in a compile error).

r328442:
Add support to the imx5/6 watchdog for the external reset signal. Also, if
the "power down" watchdog used by the ROM boot code is still active when the
regular watchdog is activated, turn off the power-down watchdog.

This adds support for the "fsl,ext-reset-output" FDT property. When
present, that property indicates that a chip reset is accomplished by
asserting the WDOG1_B external signal, which is supposed to trigger some
external component such as a PMIC to ready the hardware for reset (for
example, adjusting voltages from idle to full-power levels), and assert the
POR signal to SoC when ready. To guard against misconfiguation leading to a
non-rebootable system, the external reset signal is backstopped by code
that asserts a normal internal chip reset if nothing responds to the
external reset signal within one second.