arm: exynos_defconfig: remove CONFIG_MMC_DW_IDMAC

DesignWare MMC Controller's transfer mode should be decided
at runtime instead of compile-time. So we remove this config
option and read dw_mmc's register to select DMA master.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>