usb: phy: qusb2: Add regulator support which powers refgen

Since QUSB2PHY take biases from REFGEN it is necessary to
power refgen for QUSB2PHY to operate. This change adds regulator
support which powers the refgen. The regulator handle needs to be
passed from device tree.

Change-Id: Icd17252103d5fbc90416020ba452f303c01f1f65
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>