lvgl: change the prefix of Zephyr-specific LVGL config options

We're now using the Kconfig copied from the upstream lvgl repository. It
uses the LV_ prefix for all options while we're using LVGL_ for
Zephyr-specific ones. Make the latter consistent with upstream but also
make sure they're distinct from lvgl's by using LV_Z_ as the prefix.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>