dt-bindings: memory-controller: Define fallback compatible

dt-bindings: memory-controller: Define fallback compatible

All of the DDR controllers beyond revision b.2.1 have had a consistent
layout, therefore define a "brcm,brcmstb-memc-ddr-rev-b.2.1" fallback
compatible string to match them all rather than having to continuously
add to the list.

Link: https://lore.kernel.org/all/20241217194439.929040-2-florian.fainelli@broadcom.com/
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250609212356.2264244-2-florian.fainelli@broadcom.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>