mfd: cros_ec: Add USBPD charger commands and struct definitions.

The USBPD charger driver gets information from the ChromeOS EC, this
patch adds the USBPD charger definitions needed by this driver.

Signed-off-by: Sameer Nanda <snanda@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>