watchdog: pnx4008: convert driver to use the watchdog framework

Make this driver a user of the watchdog framework and remove parts now handled
by the core. Tested on a custom lpc32xx-board.

[wim@iguana.be: Added set_timeout operation]
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>