dt-bindings: mmc: mmc-slot: Make compatible property optional

dt-bindings: mmc: mmc-slot: Make compatible property optional

The compatible property is required for some vendor-specific device trees,
such as Amlogic's meson-mx-sdio, but is unnecessary for others, like
Microchip's DTS. To resolve unintended warnings for configurations that do
not require it, remove compatible from the required properties in
mmc-slot.yaml.

Since meson-mx-sdio still requires compatible, explicitly add it to its
required list in amlogic,meson-mx-sdio.yaml.

Signed-off-by: Dharma Balasubiramani <dharma.b@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250224-mmc-slot-v4-1-231620a31e88@microchip.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>