usb: phy: introduce usb_phy_set_event interface

PHY drivers require a generic interface to handle per-PHY events.

usb_phy_set_event interface sets event to phy event.
PHY drivers call this interface for each phy event.

Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-usb@vger.kernel.org
Cc: Android Kernel Team <kernel-team@android.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Arve Hj�nnev�g <arve@android.com>
Cc: Benoit Goby <benoit@android.com>
[Original patch in Android from Todd]
Cc: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Kiran Raparthy <kiran.kumar@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>