plat/rockchip: cliam a macro to enable hdcp feature for DP

HDCP is using a binary driver, add macro PLAT_RK_DP_HDCP to make it as
an option.

Change-Id: I54ef1a3635a28e8ae56654bd1e91dfe011520a7f
Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>