usb: phy: qmp: Make AHB2PHY register requirement optional

Currently it is mandatory to have AHB2PHY base address and offset
with QMP PHY. On some of emulators, this is not required or available,
hence make this requirement as optional. Also update devicetree
documentation for this.

Change-Id: I7a68bf504968a0aaaf91d2aee807af7a91dcbbd1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>