rockchip: transform rockchip_capsule_update_board_setup into a weak function symbol

There's only one user of rockchip_capsule_update_board_setup, which is
in board.c, and only one board defines it, so instead of having a header
only for one function symbol, let's just use a weak symbol instead.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>