drivers/mmc/dw_mmc - remove extra arch specific "asm/arch/clk.h" inclusion

1. No contents of "asm/arch/clk.h" is used within "dw_mmc.c".
2. If arch doesn't have "asm/arch/clk.h" driver won't build.

Without mentioned inclusion dw_mmc driver could be built for arches
other than ARM. For ARM driver still builds without it.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Cc: Amar <amarendra.xt@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>