cmd: pinmux: depend on PINCTRL

The pinmux command uses functions pinctrl_get_pin_*(), which are missing
if PINCTRL config option is disabled.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>