omap: Fix cmdline muxing

Looks like cmdline muxing got broken at some point when we
decided to limit muxing to __init code. Currently omap_mux_entry
list is not yet initialized when we try to initialize cmdline
muxing.

Fix this by calling omap_mux_init_list() before calling
omap_mux_set_cmdline_signals().

Reported-by: Philip Balister <philip@balister.org>
Tested-by: Philip Balister <philip@balister.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>