mmc: DT: sdhci-brcmstb: Bindings document for common sdhci-brcmstb

Changes to the DT binding document to separate the BCM7425 and the
BCM7445.

A compatible string "brcm,bcm7425-sdhci" was representing the BCM7425
SDHCI host controller with all BRCMSTB SoCs including the BCM7445. Now
it should be separated because vary a bit in initialize each host
controller.

- Renames the DT binding document to common name.
- Adds a compatible string "brcm,bcm7445-sdhci" that is representing the
  BCM7445 with thereafter 28nm generation ARM based SoCs.

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>