ARM: socfpga: dts: Enable MMC support at correct place in the DT

The socfpga.dtsi explicitly enabled MMC support, but not all boards are
equiped with an MMC card. There are setups which only have QSPI NOR.
Therefore, disable the MMC support on socfpga.dtsi level and enable it
on per-board basis.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alan Tull <atull@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Olof Johansson <olof@lixom.net>
Cc: Thor Thayer <tthayer@altera.com>
Cc: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>