mmc: add nexell driver

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>