board: rockchip: Add a common ROCK Pi 4 target

Move ROCK Pi 4 specific board code from the shared evb_rk3399 target
into its own board target and update related defconfigs to use the new
TARGET_ROCKPI4_RK3399 option.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Christopher Obbard <chris.obbard@collabora.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>