rockchip: remove support for Theobroma Systems RK3368 Lion

rockchip: remove support for Theobroma Systems RK3368 Lion

No meaningful changes were made to this SoM since February 2021. Nobody
from Theobroma has booted anything recent on that product since July
2021 at the latest. The product isn't available to buy anymore and
disappeared from our website.

This product is therefore unmaintained and it would be disingenuous to
say the opposite, so drop support for RK3368 Lion.

If you're a user of Lion, feel free to revert this patch or contact our
sales/support department.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>