ARM: socfpga: dts: rename socdk board file to socdk_sdmmc

ARM: socfpga: dts: rename socdk board file to socdk_sdmmc

Rename the socfpga_arria10_socdk board file to socfpga_arria10_socdk_sdmmc
as Arria 10 devkit cannot support SDMMC and QSPI at the same time. Thus
we will need to have 2 separate board files, one for SDMMC and one for
QSPI. We also add a new base board dtsi file, socfpga_arria10_socdk.dtsi
so that we use common peripherals for each flavor of the devkits.

Add the sdmmc node to the socfpga_arria10_socdk_sdmmc.dts board file.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>