drm/vc4: Add DSI driver

The DSI0 and DSI1 blocks on the 2835 are related hardware blocks.
Some registers move around, and the featureset is slightly different,
as DSI1 (the 4-lane DSI) is a later version of the hardware block.
This driver doesn't yet enable DSI0, since we don't have any hardware
to test against, but it does put a lot of the register definitions and
code in place.

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