usb: phy: ab8500-usb: enable/disable regulator on phy events

Add ab8500_usb_regulator_{enable,disable} functions to control USB phy
regulators on corresponding ab8500_usb_phy_{enable,disable} events.

This contains some workaround and optimization for specific AB8500
versions.

Signed-off-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Signed-off-by: Sakethram Bommisetti <sakethram.bommisetti@stericsson.com>
Signed-off-by: Praveena Nadahally <praveen.nadahally@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>