[WATCHDOG] add Nano 7240 driver

Adds support for the built-in watchdog on EPIC Nano 7240 boards from IEI.

Tested on Nano-7240RS.

Hardware documentation of the platform (including watchdog) can be found
on the IEI website: http://www.ieiworld.com

Signed-off-by: Gilles Gigan <gilles.gigan@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>