phy: Add driver for ST-Ericsson AB8500 USB PHY

The AB8500 PMIC contains an USB PHY that needs to be set up in
device or host mode to make USB work properly. Add a simple driver
for the generic PHY uclass that allows enabling it.

The if (CONFIG_IS_ENABLED(USB_MUSB_HOST)) might be a bit strange.
The USB PHY must be configured in either host or device mode and
somehow the USB PHY driver must be made aware of the mode.

Actually, the MUSB driver used together with this PHY does not
support dynamic selection of host/device mode in U-Boot at the moment.
Therefore, one very simple approach that works fine is to select
the mode to configure at compile time. When the MUSB driver is
configured in host mode the PHY is configured in host mode, and
similarly when the MUSB driver is configured in device/gadget mode.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>