Add SDMMC support

[thejpster]

I'd like to use the microSD slot on the STM32F7-Discovery board. This is connected to:

Pin	GPIO	Description
A12	PC12	SDMMC_CK
D12	PD2	SDMMC_CMD
G14	PC8	SDMMC_D0
F14	PC9	SDMMC_D1
B14	PC10	SDMMC_D2
B13	PC11	SDMMC_D3

I have an embedded-sdmmc crate, but that only handles SD cards connected to generic embedded-hal::spi devices. I'd like to add an implementation of the embedded-sdmmc::blockdevice::BlockDevice for the SDMMC peripheral on the STM32F7.

I'll try a few things out in my fork and send a PR once I think it's working.

[systec-ms]

I've been working on getting SD/SDIO/MMC card host interface (SDMMC) to work, didn't get done with it though until priorities shifted. Maybe someone would like to build on my previous work (which is predominantly on customizations of the stm32h7 hal implementation).
The reading worked partially.
https://github.com/systec-ms/stm32f7xx-hal/tree/sdmmc