mmc: openpiton: add piton_mmc driver

This commit adds support to piton_mmc driver for OpenPiton-riscv64
This driver has many things set as preconfigured because the hardware
automatically configures most of the settings during startup.

Signed-off-by: Tianrui Wei <tianrui-wei@outlook.com>
Signed-off-by: Jonathan Balkind <jbalkind@ucsb.edu>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 89d6af3..d96ac90 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -72,6 +72,7 @@
 obj-$(CONFIG_MMC_SDHCI_ZYNQ)		+= zynq_sdhci.o
 
 obj-$(CONFIG_MMC_SUNXI)			+= sunxi_mmc.o
+obj-$(CONFIG_MMC_PITON)			+= piton_mmc.o
 obj-$(CONFIG_MMC_UNIPHIER)		+= tmio-common.o uniphier-sd.o
 obj-$(CONFIG_RENESAS_SDHI)		+= tmio-common.o renesas-sdhi.o
 obj-$(CONFIG_MMC_BCM2835)		+= bcm2835_sdhost.o