board: rockchip: rock5b-rk3588: fix description

Update description with correct specifications

Fixes: 3bf8e408 ("board: rockchip: add Radxa ROCK5B Rk3588 board")
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>