omap: mux: fix multipath gpio handling

OMAP3530 CBB package can have GPIO126 muxed on 2 pins: mmc1_dat4 and
cam_strobe. This causes a problem with current multipath GPIO mux
handling, which muxes both pins as GPIO126 and makes the GPIO unusable.

Fix this by not muxing any pins if multipath GPIO is detected and
just print a warning instead. It's up to board files to set correct
mux using omap_mux_init_signal and pin name.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>