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.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/Kconfig

@@ -1094,3 +1094,16 @@ config MMC_OWL
 
 config MMC_SDHCI_EXTERNAL_DMA
 	bool
+
+config MMC_LITEX
+	tristate "LiteX MMC Host Controller support"
+	depends on ((PPC_MICROWATT || LITEX) && OF && HAVE_CLK) || COMPILE_TEST
+	select REGULATOR
+	select REGULATOR_FIXED_VOLTAGE
+	help
+	  This selects support for the MMC Host Controller found in LiteX SoCs.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called litex_mmc.
+
+	  If unsure, say N.

drivers/mmc/host/Makefile

@@ -101,6 +101,7 @@ obj-$(CONFIG_MMC_CQHCI)			+= cqhci.o
 cqhci-y					+= cqhci-core.o
 cqhci-$(CONFIG_MMC_CRYPTO)		+= cqhci-crypto.o
 obj-$(CONFIG_MMC_HSQ)			+= mmc_hsq.o
+obj-$(CONFIG_MMC_LITEX)			+= litex_mmc.o
 
 ifeq ($(CONFIG_CB710_DEBUG),y)
 	CFLAGS-cb710-mmc	+= -DDEBUG