modules: lvgl: Fix usage of LVGL log levels

Introduces a Kconfig symbol `LV_Z_LOG_LEVEL` because contrary to Zephyr the
numerical value of log levels in LVGL increases with severity. Also support
for the `LV_LOG_LEVEL_USER` is added.

Resolves issue #64351.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>