drm/meson: add RDMA register bits defines

The Amlogic 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.

This adds the register defines.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021091509.3864-3-narmstrong@baylibre.com