dt-bindings: mmc: cdns: document Microchip MPFS MMC/SDHCI controller

The Microchip PolarFire SoC FPGA DTSI uses Cadence SD/SDIO/eMMC Host
Controller without any additional vendor compatible:

  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: mmc@20008000: compatible:0: 'cdns,sd4hc' is not one of ['socionext,uniphier-sd4hc']
  arch/riscv/boot/dts/microchip/microchip-mpfs-icicle-kit.dt.yaml: mmc@20008000: compatible: ['cdns,sd4hc'] is too short

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>