doc: mmc dev

Provide human readable descriptions of the speed nodes instead of the name
of constants from the code as it is already done for 'mmc rescan'
command in commit 212f0784 ("doc: mmc rescan speed mode").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>