dt-bindings: phy: List supplies for qcom,edp-phy

We're supposed to list the supplies in the dt bindings but there are
none in the eDP PHY bindings.

Looking at the driver in Linux, I can see that there seem to be two
relevant supplies: "vdda-phy" and "vdda-pll". Let's add those to the
bindings.

NOTE: from looking at the Qualcomm datasheet for sc7280, it's not
immediately clear how to figure out how to fill in these supplies. The
only two eDP related supplies are simply described as "power for eDP
0.9V circuits" and "power for eDP 1.2V circuits". From guessing and
from comparing how a similar PHY is hooked up on other similar
Qualcomm boards, I'll make the educated guess that the 1.2V supply
goes to "vdda-phy" and the 0.9V supply goes to "vdda-pll" and I'll use
that in the example here.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sankeerth Billakanti <quic_sbillaka@quicinc.com>
Link: https://lore.kernel.org/r/20220425140619.2.Iae013f0ff4599294189f3a6e91376fad137bbabf@changeid
Signed-off-by: Vinod Koul <vkoul@kernel.org>