clk: socfpga: Fix incorrect sdmmc clock name

The SD/MMC clock is named "sdmmc_clk", and NOT "mmc_clk". Because of this,
the SD driver was getting the incorrect clock value. This prevented the
SD driver from initializing correctly.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Olof Johansson <olof@lixom.net>
Reviewed-by: Pavel Machek <pavel@denx.de>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Mike Turquette <mturquette@linaro.org>