doc: mmc rescan speed mode

Provide human readable descriptions of the speed nodes instead of the name
of constants from the code.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/mmc.rst

@@ -51,22 +51,26 @@ The 'mmc rescan' command scans the available MMC device.
 
    mode
        speed mode to set.
-       CONFIG_MMC_SPEED_MODE_SET should be enabled. The required speed mode is
-       passed as the index from the following list.
-
-       0   - MMC_LEGACY
-       1   - MMC_HS
-       2   - SD_HS
-       3   - MMC_HS_52
-       4   - MMC_DDR_52
-       5   - UHS_SDR12
-       6   - UHS_SDR25
-       7   - UHS_SDR50
-       8   - UHS_DDR50
-       9   - UHS_SDR104
-       10  - MMC_HS_200
-       11  - MMC_HS_400
-       12  - MMC_HS_400_ES
+       CONFIG_MMC_SPEED_MODE_SET should be enabled. The requested speed mode is
+       passed as a decimal number according to the following table:
+
+       ========== ==========================
+       Speed mode Description
+       ========== ==========================
+           0      MMC legacy
+           1      MMC High Speed (26MHz)
+           2      SD High Speed (50MHz)
+           3      MMC High Speed (52MHz)
+           4      MMC DDR52 (52MHz)
+           5      UHS SDR12 (25MHz)
+           6      UHS SDR25 (50MHz)
+           7      UHS SDR50 (100MHz)
+           8      UHS DDR50 (50MHz)
+           9      UHS SDR104 (208MHz)
+          10      HS200 (200MHz)
+          11      HS400 (200MHz)
+          12      HS400ES (200MHz)
+       ========== ==========================
 
        A speed mode can be set only if it has already been enabled in the device tree