usb: phy: ab8500-usb: add ab8540 support

Add support for the ab8540 variant of the ab8500 family.

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