ARM: exynos_defconfig: compile Atmel MXT touchscreeen as module

The Atmel MXT touchscreen can load firmware and settings from the
/lib/firmware directory, it makes sense to have it as a module to have
more control over the loading process.

Signed-off-by: Martin Jücker <martin.juecker@gmail.com>
Link: https://lore.kernel.org/r/20201113212525.13455-5-martin.juecker@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>