cmd: mmc: Add mmc reg read command for reading card registers

Add extension to the 'mmc' command to read out the card registers.
Currently, only the eMMC OCR/CID/CSD/EXTCSD/RCA/DSR register are
supported. A register value can either be displayed or read into
an environment variable.

Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 60aed2c..fd16c3a 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1313,6 +1313,13 @@
 	  on a eMMC device. The feature is optionally available on eMMC devices
 	  conforming to standard >= 4.41.
 
+config CMD_MMC_REG
+	bool "Enable support for reading card registers in the mmc command"
+	depends on CMD_MMC
+	help
+	  Enable the commands for reading card registers. This is useful
+	  mostly for debugging or extracting details from the card.
+
 config CMD_MMC_RPMB
 	bool "Enable support for RPMB in the mmc command"
 	depends on SUPPORT_EMMC_RPMB