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>