[WATCHDOG] Convert mtx1 wdt to be a platform device and use generic GPIO API

This patch converts the MTX-1 to be a platform device, use the available
generic GPIO API for the MTX-1 board and register the miscdev alias.

Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>