drm/v3d: Add support for V3D v4.2.

No compatible string for it yet, just the version-dependent changes.
They've now tied the hub and the core interrupt lines into a single
interrupt line coming out of the block.  It also turns out I made a
mistake in modeling the V3D v3.3 and v4.1 bridge as a part of V3D
itself -- the bridge is going away in favor of an external reset
controller in a larger HW module.

v2: Use consistent checks for whether we're on 4.2, and fix a leak in
    an error path.
v3: Use more general means of determining if the current 4.2 changes
    are in place, as apparently other platforms may switch back (noted
    by Dave).  Update the binding doc.

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