ARM: rockchip: remove cpu-core name from machine name

The Rockchip support is not limited to Cortex-A9 socs anymore and its
presence may confuse people reading /proc/cpuinfo. So remove the core
specific part.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>