rockchip: block: blk-uclass: add bounce buffer flag to blk_desc

Currently bounce buffer support is enabled for all block devices
when available. Add a flag to blk_desc to enable only on demand.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>