drm/vc4: Add devicetree bindings for VC4.

VC4 is the GPU (display and 3D) subsystem present on the 2835 and some
other Broadcom SoCs.

This binding follows the model of msm, imx, sti, and others, where
there is a subsystem node for the whole GPU, with nodes for the
individual HW components within it.

v2: Extend the commit message, fix several nits from Stephen Warren.
v3: Rename the compatibility strings, clean up node names, drop the
    unnecessary lists of components.  Use compatibility strings for
    choosing CRTC HVS channel numbers.  Document the HDMI clock usage.
v4: Whitespace fix, expand acronyms, move to display/ instead of gpu/,
    rename "hpd-gpio" to "hpd-gpios".

Signed-off-by: Eric Anholt <eric@anholt.net>