modules: lvgl: define the Kconfig types used in boards

Some boards override default for few LVGL LV_Z_* symbols. This causes
the build to fail if the module is not present in the system, and has
been worked around on one board by repeating the types in the board
Kconfig file.

Fix this properly by defining the known type for all symbols used in
boards in modules/Kconfig.lvgl.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>