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>
mfd: rn5t618: document device tree bindings

This adds the device tree bindings documentation for Ricoh RN5T618.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Mark Brown <broonie@linaro.org>
ARM: dts: meson6-atv1200: add rn5t618 mfd node

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>