cmd: pinmux: Add pinmux command

pinmux command allows to :
 - list all pin-controllers available on platforms
 - select a pin-controller
 - display the muxing of all pins of the current pin-controller
   or all pin-controllers depending of given options

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
cmd: pinmux: Fix pinmux command

if "pinmux status" command is used without having
set dev using "pinmux dev", print pinmux usage
Reviewed-by: Simon Glass <sjg@chromium.org>