drm/meson: add RDMA module driver

The VPU embeds a "Register DMA" that can write a sequence of registers
on the VPU AHB bus, either manually or triggered by an internal IRQ
event like VSYNC or a line input counter.

The initial implementation handles a single channel (over 8), triggered
by the VSYNC irq and does not handle the RDMA irq.

The RDMA will be usefull to reset and program the AFBC decoder unit
on each vsync without involving the interrupt handler that can
be masked for a log period of time, producing display glitches.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021091509.3864-5-narmstrong@baylibre.com