platform/chrome: cros_usbpd_notify: Listen to EC_HOST_EVENT_USB_MUX host event

platform/chrome: cros_usbpd_notify: Listen to EC_HOST_EVENT_USB_MUX host event

On system that use ACPI, cros_usbpd_notify gets notifications of USB MUX
host event same as PD host events [1]. But currently on system that use
DT, the driver only listen on EC_HOST_EVENT_PD_MCU.

Add EC_HOST_EVENT_USB_MUX to the list of host events, so we have same
behavior on all platforms.

[1]: https://chromium.googlesource.com/chromiumos/third_party/coreboot/+/refs/heads/chromeos-2016.05/src/ec/google/chromeec/acpi/ec.asl#382

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210414064524.2450908-1-pihsun@chromium.org