boards: Add lvgl pointer support on m5stack_core2

Added lvgl-pointer definition in devicetree, to enable
touch-support in lvgl applications.
Kscan support has been removed from m5stack_core2 as this was just a
temporary thing.
Has been tested with lvgl hello world sample.

Signed-off-by: Martin Kiepfer <m.kiepfer@teleschirm.org>