phy: core: Support regulator supply for PHY power

Some PHYs can be powered by an external power regulator.
e.g. USB_HS PHY on DRA7 SoC. Make the PHY core support a
power regulator.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>