watchdog: digicolor: driver for Conexant Digicolor CX92755 SoC

This commit add a driver for the watchdog functionality of the Conexant CX92755
SoC, from the Digicolor series of SoCs. Of 8 system timers provided by the
CX92755, the first one, timer A, can reset the chip when its counter reaches
zero. This driver uses this capability to provide userspace with a standard
watchdog, using the watchdog timer driver core framework. This driver also
implements a reboot handler for the reboot(2) system call.

The watchdog driver shares the timer registers with the CX92755 timer driver
(drivers/clocksource/timer-digicolor.c). The timer driver, however, uses only
timers other than A, so both drivers should coexist.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>