imx: mx6slevk: enable more DM drivers

Enable more DM drivers. The imx I2C/MMC DM drivers needs DM_GPIO
enabled. So needs to enable them together.

DM FEC and SPI are not enabled, but they use gpio in board code.
So use gpio_request first to request the gpio, because DM_GPIO
is enabled.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>