omap: Fix undefined reference to omap2_i2c_mux_pins

In some cases we can get error function `omap2_i2c_add_bus':
arch/arm/plat-omap/i2c.c:136: undefined reference to `omap2_i2c_mux_pins'
arch/arm/plat-omap/i2c.c:141: undefined reference to `omap_hwmod_lookup'
arch/arm/plat-omap/i2c.c:157: undefined reference to `omap_device_build'

Signed-off-by: Tony Lindgren <tony@atomide.com>