samples: display: lvgl: add min_flash/min_ram

LVGL eats a considerable amount of flash, so tests may fail on platforms
that meet all requirements but that have a low amount of flash, e.g.
nucleo_g071rb. Set min_flash for all cases to 250K (result of some quick
experimentation). Also update minimum ram to exclude platforms that
cannot run Zephyr + LVGL (e.g. those with just 16K).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>