mmc: dw_mmc-pltfm: Remove unused <linux/clk.h>

As of commit 4cdc2ec1

 ("mmc: dw_mmc: move rockchip related code
to a separate file"), dw_mmc-pltfm.c no longer uses the clock API.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20210527194226.1705607-1-geert@linux-m68k.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>