mmc

This patch adds SD/MMC support for STM32H7 SoCs.

Here is an extraction of SDMMC main features, embedded in
STM32H7 SoCs.
The SD/MMC block include the following:
 _ Full compliance with MultiMediaCard System Specification
   Version 4.51. Card support for three different databus modes:
   1-bit (default), 4-bit and 8-bit.
 _ Full compatibility with previous versions of MultiMediaCards
   (backward compatibility).
 _ Full compliance with SD memory card specifications version 4.1.
   (SDR104 SDMMC_CK speed limited to maximum allowed IO speed,
    SPI mode and UHS-II mode not supported).
 _ Full compliance with SDIO card specification version 4.0.
   Card support for two different databus modes: 1-bit (default)
   and 4-bit. (SDR104 SDMMC_CK speed limited to maximum allowed IO
   speed, SPI mode and UHS-II mode not supported).
 _ Data transfer up to 208 Mbyte/s for the 8 bit mode.
   (depending maximum allowed IO speed).
 _ Data and command output enable signals to control external
   bidirectional drivers.

The current version of the SDMMC supports only one SD/SDIO/MMC card
at any one time and a stack of MMC Version 4.51 or previous.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>