ARM: mxs_defconfig: Build CONFIG_INPUT_EVDEV as built-in

Build CONFIG_INPUT_EVDEV as a built-in driver, as it makes easier to test
the LRADC touchscreen driver, for example, by using the 'evtest' tool.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>