usb: phy: qusb2: Add support to vote for regulator L2a

L2a is required to lock the phy PLL upon bus resume when
exiting from XO shutdown. This LDO powers REFGEN block
which is required to be powered on so that phy PLL gets
locked as part of wakeup from XO shutdown.

Change-Id: Ia0e3d574de7c78534832e4f8749672eb6fcde1f0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>