This adds a rockchip.rst to docs/plat documenting the general
approach to using the Rockchip ATF platforms together with the
supported bootloaders and also adds myself as maintainer after
making sure Tony Xie is ok with that.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: Idce53d15eff4ac6de05bbb35d86e57ed50d0cbb9