dt-bindings: soc: ti: k3-ringacc: Describe cfg reg region

dt-bindings: soc: ti: k3-ringacc: Describe cfg reg region

RINGACC module on K3 SoCs have CFG register region which is 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>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230809175932.2553156-2-vigneshr@ti.com
Signed-off-by: Nishanth Menon <nm@ti.com>