arm: socfpga: Remove cpu_mmc_init()

This function triggers the registration of the dwmmc driver on SoCFPGA,
but this is not needed in case the driver is correctly probed from DT.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>