dt-bindings: dma: ti: k3-udma: Describe cfg register regions

dt-bindings: dma: ti: k3-udma: Describe cfg register regions

Unified DMA (UDMA) module on K3 SoCs have TX and RX channel cfg and RX
flow cfg register regions which are usually configured by a Device
Management firmware. But certain entities such as bootloader (like
U-Boot) may have to access them directly. Describe this region in the
binding documentation for completeness of module description.

Keep the binding compatible with existing DTS files by requiring first
four regions to be present at least.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231124045722.191817-5-vigneshr@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>