lib: lvgl: obtain LVGL_DISPLAY_DEV_NAME from devicetree

It is not possible completely remove options like
LVGL_DISPLAY_DEV_NAME without heavy reworking of LVGL support
because lvgl_init needs the name of the device, and
LVGL_*_RES_MAX options to define internal buffers.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>