powerpc/watchdog: Allow the Book-E driver to be compiled as a module

Register the __init and __exit functions in the PowerPC Book-E Watchdog
driver as module entry/exit functions, and modify the Kconfig entry.

Add a .release method for the PowerPC Book-E Watchdog driver, so that the
watchdog is disabled when the driver is closed.

Loosely based on original code from Jiang Yutang <b14898@freescale.com>.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>