ARM: OMAP3: Fix omap3evm randconfig error introduced by VBUS support

Commit cb8ca589 (ARM: omap3evm: enable VBUS switch for EHCI tranceiver)
added a new randconfig error if TWL4030_CORE is not selected:

arch/arm/mach-omap2/board-omap3evm.c:368: undefined reference to `twl_i2c_read_u8'
arch/arm/mach-omap2/board-omap3evm.c:370: undefined reference to `twl_i2c_write_u8'

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