boards: stm32n6570_dk: Remove invert-y property from LVGL pointer input

Removed the invert-y property from the LVGL pointer input configuration
as it's causing the touch events to be "flipped" vertically.

Confirmed that this gets samples/subsys/display/lvgl back to correctly
detecting touch events, and for good measure tested
samples/subsys/input/draw_touch_events which keeps working as expected
(like it should since it's not using the LVGL pointer input ;-))

Fixes zephyrproject-rtos/zephyr#98933

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>