dt-bindings: mmc: npcm,sdhci: Document NPCM SDHCI controller

Add binding for Nuvoton NPCM SDHCI controller.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231002200610.129799-2-tmaimon77@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>