mmc: Kconfig: Add RISCV and CSKY for MMC_DW

Synopsys DesignWare MMC controller could be used in RISC-V and
C-SKY architectures.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/1596887102-9743-1-git-send-email-guoren@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>