ARM: legoev3: convert to driver model

This converts LEGO MINDSTORMS EV3 to the driver model. MMC, SERIAL, SPI
and SPI_FLASH are converted.

The device tree contains only the minimal nodes required by U-Boot
since the size of U-Boot is limited to 256K on this device.

Signed-off-by: David Lechner <david@lechnology.com>