dt-bindings: phy: Clarify ULPI PHY source clock

cdev2 is not actually a clock on Tegra20 but rather a pinmux pad group.
PLL_P_OUT4 is the source clock for the ULPI PHY and is output to the
DAP_MCLK2 pad.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>