MMC/SD/SDIO slot directly connected to a SPI bus

Required properties:
- compatible : should be "mmc-spi-slot".
- reg : should specify SPI address (chip-select number).
- spi-max-frequency : maximum frequency for this device (Hz).
- voltage-ranges : two cells are required, first cell specifies minimum
  slot voltage (mV), second cell specifies maximum slot voltage (mV).
  Several ranges could be specified.
- gpios : (optional) may specify GPIOs in this order: Card-Detect GPIO,
  Write-Protect GPIO.

Example:

	mmc-slot@0 {
		compatible = "fsl,mpc8323rdb-mmc-slot",
			     "mmc-spi-slot";
		reg = <0>;
		gpios = <&qe_pio_d 14 1
			 &qe_pio_d 15 0>;
		voltage-ranges = <3300 3300>;
		spi-max-frequency = <50000000>;
	};
