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/Makefile b/drivers/mmc/Makefile
index e84c792..d375669 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -44,6 +44,7 @@
 obj-$(CONFIG_SH_SDHI) += sh_sdhi.o
 obj-$(CONFIG_STM32_SDMMC2) += stm32_sdmmc2.o
 obj-$(CONFIG_JZ47XX_MMC) += jz_mmc.o
+obj-$(CONFIG_NEXELL_DWMMC) += nexell_dw_mmc.o
 
 # SDHCI
 obj-$(CONFIG_MMC_SDHCI)			+= sdhci.o