mmc: Add driver for LiteX's LiteSDCard interface

mmc: Add driver for LiteX's LiteSDCard interface

LiteX (https://github.com/enjoy-digital/litex) is a SoC framework
that targets FPGAs. LiteSDCard is a small footprint, configurable
SDCard core commonly used in LiteX designs.

The driver was first written in May 2020 and has been maintained
cooperatively by the LiteX community. Thanks to all contributors!

Co-developed-by: Kamil Rakoczy <krakoczy@antmicro.com>
Signed-off-by: Kamil Rakoczy <krakoczy@antmicro.com>
Co-developed-by: Maciej Dudek <mdudek@internships.antmicro.com>
Signed-off-by: Maciej Dudek <mdudek@internships.antmicro.com>
Co-developed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220113170300.3555651-4-gsomlo@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>