ARM: OMAP2+: Fix zoom LCD backlight if TWL_CORE is not selected

Otherwise we get:

arch/arm/mach-omap2/board-zoom-display.c:64: undefined reference to `twl_i2c_read_u8'
arch/arm/mach-omap2/board-zoom-display.c:65: undefined reference to `twl_i2c_read_u8'
arch/arm/mach-omap2/board-zoom-display.c:84: undefined reference to `twl_i2c_write_u8'
arch/arm/mach-omap2/board-zoom-display.c:86: undefined reference to `twl_i2c_write_u8'
arch/arm/mach-omap2/board-zoom-display.c:91: undefined reference to `twl_i2c_write_u8'
arch/arm/mach-omap2/board-zoom-display.c:92: undefined reference to `twl_i2c_write_u8'
arch/arm/mach-omap2/board-zoom-display.c:72: undefined reference to `twl_i2c_write_u8'

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