ARM: multi_v7_defconfig: Build Atmel maXTouch driver as a module

The driver is for a trackpad device so is not needed for booting and
makes more sense to have it as module to reduce the kernel image size.

It was probably enabled as built-in because module autoload was not
working when the I2C device was registered by OF but this got fixed
in commit b7d21058 ("Input: atmel_mxt_ts - add maxtouch to I2C
table for module autoload") so it's safe to enable as a module now.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>