dt-bindings: memory-controller: Document Broadcom STB MEMC

Document the Broadcom STB memory controller which is a trivial binding
for now with a set of compatible strings and single register.

Since we introduce this binding, the section in
Documentation/devicetree/bindings/arm/bcm/brcm,brcmstb.txt is removed
and this binding is referenced instead.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
[krzk: correct path in brcm,brcmstb.txt]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220812222533.2428033-2-f.fainelli@gmail.com