ARM: dts: msm: add HDMI device for msm8994 platform

Add HDMI device for 20nm HDMI PHY on msm8994 platform,
with all required bindings. As hpd gpio is now an
optional binding, update corresponding binding
documentation. Also add support for external framebuffer.

Change-Id: Ifa13e2e9f8cdc6c84dd62df0cd217ba13e58deac
Signed-off-by: Casey Piper <cpiper@codeaurora.org>