power_supply: rt9455_charger: Check if CONFIG_USB_PHY is enabled

If CONFIG_USB_PHY is not enabled, struct notifier_block is not defined and
compilation fails. Therefore, the functions that process USB event
notifications are defined only if CONFIG_USB_PHY is enabled.
There is no need to define these functions if CONFIG_USB_PHY is not
enabled, since no USB notifications are received in this case.
Also, since rt9455_set_boost_voltage_before_boost_mode() function is
called only if USB_EVENT_ID notification is received, this function should
also be defined only if CONFIG_USB_PHY is enabled.

Signed-off-by: Anda-Maria Nicolae <anda-maria.nicolae@intel.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>