mmc: move CONFIG_DWMMC to Kconfig, renaming to CONFIG_MMC_DW

This commit was created as follows:

[1] Rename the option with the following command:
find . -name .git -prune -o ! -path ./scripts/config_whitelist.txt \
-type f -print | xargs sed -i -e 's/CONFIG_DWMMC/CONFIG_MMC_DW/g'

[2] create the entry for MMC_DW in drivers/mmc/Kconfig
    (the prompt and help were copied from Linux)

[3] run "tools/moveconfig.py -y MMC_DW"

[4] add "depends on MMC_DW" to the MMC_DW_ROCKCHIP entry

[5] Clean-up doc/README.socfpga by hand

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/configs/firefly-rk3288_defconfig b/configs/firefly-rk3288_defconfig
index 3e55a84..eb78d54 100644
--- a/configs/firefly-rk3288_defconfig
+++ b/configs/firefly-rk3288_defconfig
@@ -43,6 +43,7 @@
 CONFIG_SYS_I2C_ROCKCHIP=y
 CONFIG_LED=y
 CONFIG_LED_GPIO=y
+CONFIG_MMC_DW=y
 CONFIG_MMC_DW_ROCKCHIP=y
 CONFIG_PINCTRL=y
 CONFIG_SPL_PINCTRL=y