mx6slevk: Fix MMC breakage for the SPL target

Commit 001cdbbb ("imx: mx6slevk: enable more DM drivers") breaks
MMC support in U-Boot proper on the mx6slevk_spl_defconfig target:

U-Boot SPL 2017.09-00396-g6ca43a5 (Oct 01 2017 - 16:20:18)
Trying to boot from MMC1

U-Boot 2017.09-00396-g6ca43a5 (Oct 01 2017 - 16:20:18 -0300)

CPU:   Freescale i.MX6SL rev1.0 792 MHz (running at 396 MHz)
CPU:   Commercial temperature grade (0C to 95C) at 33C
Reset cause: POR
Board: MX6SLEVK
I2C:   ready
DRAM:  1 GiB
MMC:   FSL_SDHC: 0
MMC Device 1 not found
*** Warning - No MMC card found, using default environment

As mx6slevk_spl_defconfig does not use CONFIG_DM_MMC and its
board file does not register the mmc controller for U-Boot proper,
let's fix this by adding CONFIG_DM_MMC=y and device tree support.

While at it, add more DM drivers, so that it becomes closer to
mx6slevk_defconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>