mfd: cros_ec: Check DT node for usbpd-notify add

Add a check to ensure there is indeed an EC device tree entry before
adding the cros-usbpd-notify device. This covers configs where both
CONFIG_ACPI and CONFIG_OF are defined, but the EC device is defined
using device tree and not in ACPI.

Fixes: 4602dce0 ("mfd: cros_ec: Add cros-usbpd-notify subdevice")
Signed-off-by: Prashant Malani <pmalani@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>