mmc: add nexell driver

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- driver changed to DM.
- pinctrl-driver/dt is used now instead of configuring the mmc I/O-pins
  in the mmc-driver.
- nexell_dwmmc_ofdata_to_platdata() reworked, i.e. valid default values
  are used now (where possible) and the appropriate if-blocks have
  been removed.
- new dt-property "mmcboost" is used now instead of "CONFIG_BOOST_MMC"
  which was not defined anywhere.

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>
diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index ad86c23..556b3ac 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -263,6 +263,14 @@
 	  This selects support for Synopsys DesignWare Memory Card Interface driver
 	  extensions used in various Synopsys ARC devboards.
 
+config NEXELL_DWMMC
+	bool "Nexell SD/MMC controller support"
+	depends on ARCH_NEXELL
+	depends on MMC_DW
+	depends on DM_MMC
+	depends on PINCTRL_NEXELL
+	default y
+
 config MMC_MESON_GX
 	bool "Meson GX EMMC controller support"
 	depends on DM_MMC && BLK && ARCH_MESON