watchdog.h: Move hw_watchdog_init prototype to end, guard

In commit abbab703 we added a prototype for hw_watchdog_init, in a
Freescale i.MX watchdog driver.  We did not however guard it with an
__ASSEMBLY__ check and broke some platforms.  Move this to the end of
the file with other prototypes and guard it.

Signed-off-by: Tom Rini <trini@ti.com>