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>

doc/usage/cmd/mmc.rst

@@ -21,6 +21,7 @@ Synopsis
     mmc bootpart-resize <dev> <dev part size MB> <RPMB part size MB>
     mmc partconf <dev> [[varname] | [<boot_ack> <boot_partition> <partition_access>]]
     mmc rst-function <dev> <value>
+    mmc reg read <reg> <offset> [env]
 
 Description
 -----------
@@ -183,6 +184,31 @@ The 'mmc rst-function' command changes the RST_n_FUNCTION field.
         0x3
             Reserved
 
+The 'mmc reg read <reg> <offset> [env]' reads eMMC card register and
+either print it to standard output, or store the value in environment
+variable.
+
+<reg> with
+optional offset <offset> into the register array, and print it to
+standard output or store it into environment variable [env].
+
+    reg
+        cid
+            The Device IDentification (CID) register. Uses offset.
+        csd
+            The Device-Specific Data (CSD) register. Uses offset.
+        dsr
+            The driver stage register (DSR).
+        ocr
+            The operation conditions register (OCR).
+        rca
+            The relative Device address (RCA) register.
+        extcsd
+            The Extended CSD register. Uses offset.
+    offset
+        For 'cid'/'csd' 128 bit registers '[0..3]' in 32-bit increments. For 'extcsd' 512 bit register '[0..512,all]' in 8-bit increments, or 'all' to read the entire register.
+    env
+        Optional environment variable into which 32-bit value read from register should be stored.
 
 Examples
 --------