phy: keystone-usb: handle the transition of the USB power domain

There is no proper power domain support for the keystone platforms.
However we need to turn off the USB domains before jumping to linux or it
fail to boot (observed with k2e and k2l platforms).
This can be done in the PHY driver as it is dedicated only to the keystone
platforms and matches the required on/off sequence.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>