plat/rockchip: Use new bl31_params_parse_helper()

The Rockchip platform is a prime candidate for switching to the new
bl31_params_parse_helper(), so switch it over. This will allow BL2
implementations on this platform to transparently switch over to the
version 2 parameter structure.

Change-Id: I540741d2425c93f66c8697ce749a351eb2b3a7e8
Signed-off-by: Julius Werner <jwerner@chromium.org>