ARM: omap: fix section mismatch warning in mux.c

WARNING: arch/arm/mach-omap2/built-in.o(.text+0x15a4): Section mismatch in reference from the function omap_mux_init_signals() to the function .init.text:omap_mux_init_signal()
The function omap_mux_init_signals() references
the function __init omap_mux_init_signal().
This is often because omap_mux_init_signals lacks a __init
annotation or the annotation of omap_mux_init_signal is wrong.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>