watchdog: add driver for Ricoh RN5T618 watchdog

This adds a driver for the watchdog timer available in Ricoh RN5T618
PMIC. The device supports a programmable expiration time of 1, 8, 32
or 128 seconds.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

diff --cc drivers/watchdog/Kconfig
Simple merge

diff --cc drivers/watchdog/Makefile
Simple merge

diff --cc drivers/watchdog/rn5t618_wdt.c
Simple merge