usb: phy: ab8500-usb: add flag bits to control driver features

Introduce a "flags" field in "struct ab8500_usb" to allow controlling
driver features and quirks depending on ab8500 chip variant and
revision.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>