rockchip: Move bootrom helper compilation to a hidden option

Right now the ROCKCHIP_SPL_BACK_TO_BROM option both triggers
compilation of the bootrom hook-code as well as enabling the
behaviour of loading the full U-Boot via the boot.

New added socs may always need the bootrom code, while still
being able to decide between loading U-Boot regularly or via
the bootrom separately.

So move the compilation of the bootrom code to a hidden option
that gets selected by ROCKCHIP_SPL_BACK_TO_BROM, but can also
be selected by other parts.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>