usb: phy: qmp: Make vls_clamp_reg optional

On newer QMP USB3 UNI PHYs, the VLS clamp register is included
as part of the PCS registers instead of a TCSR. Since there is
already a separate register added for it, it may not be required
to also write to the TCSR, so hence make it an optional DT binding.

While at it, remove emulation support as it is not currently used
nor needed. Also remove the redundant qmp_phy_reg_offset member
from struct msm_ssphy_qmp as phy_reg is already being used.

Change-Id: If343e4a674b8aff8531e334e6ff74ad068544b0a
Signed-off-by: Jack Pham <jackp@codeaurora.org>