ARM: dts: msm: add required extpclk for HDMI on msm8994

Add the extpclk that is needed for HDMI interface on
msm8994. This binds the clock exposed by pll driver
to the client clock used by the driver.

Change-Id: I5ff2811e89d3adac50b1d97d740d402550332709
Signed-off-by: Casey Piper <cpiper@codeaurora.org>