rockchip: make the global board_fit_config_name_match __weak

The core Rockchip spl code contains a default board_fit_config_name_match
implementation doing nothing. Individual boards may want to handle this
differently, so add a __weak atribute to make it possible to override
this function in other places.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>