ARM: omap2: set CONFIG_LIRC=y in defconfig

The CONFIG_LIRC symbol has changed from 'tristate' to 'bool, so we now
get a warning for omap2plus_defconfig:

arch/arm/configs/omap2plus_defconfig:322:warning: symbol value 'm' invalid for LIRC

This changes the file to mark the symbol as built-in to get rid of the
warning.

Fixes: a60d64b1 ("media: lirc: lirc interface should not be a raw decoder")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>