USB: Add reg_name usage with SSUSB QMP PHY

This change adds reg_name usage with SSUSB QMP PHY and makes
"qmp_ahb2phy_base" as required address set with "qmp_phy_base".
It updates devicetree documentation accordingly.

Change-Id: I6aad12d8c798ec6d987ffd998749f4368aa25598
Signed-off-by: Mayank Rana <mrana@codeaurora.org>