drm/vc4: crtc: Rename SoC data structures

Since we're going to introduce pixelvalve data structures for other SoCs
than the BCM2835, let's rename the structures defined in the code to
make it obvious which SoC we're targeting.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/39aed7dd512ce2a4560902974ec26b16b88ec68b.1590594512.git-series.maxime@cerno.tech