dt-bindings: phy: qcom-qmp: Cleanup the 'reg' documentation as per review

After the commit 8b1087fa ("phy: qcom-qmp: Fix dts bindings to
reflect reality") landed there was some review feedback that 'reg'
should have been documented differently.  Fix it as per review
feedback.

As per that feedback:
- Subject should have been 'dt-bindings: phy:' which this patch now
  has.
- We should leave no ambiguity in the ordering of 'reg' ranges even if
  'reg-names' are also specified.
- Normally using reg-names is discouraged unless there's a strong
  reason it's needed (like if there are optional ranges).  In this
  case reg-names wasn't needed but the driver already landed relying
  on reg-names so we'll just document it and move on.

Fixes: 8b1087fa

 ("phy: qcom-qmp: Fix dts bindings to reflect reality")
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>