console: add function console_devices_set

Add a new function to access to console_devices only defined if
CONFIG_IS_ENABLED(CONSOLE_MUX).

This path allows to remove #if CONFIG_IS_ENABLED(CONSOLE_MUX)
in console_getc function.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>