clk: rockchip: rename rga clock-id on rk3399

The rga clock supplying the working clock on the rk3399 is actually
called rga-core in the manual. As the clock id has neither been
assigned nor released with a full kernel release, we can still change
the id to the more appropriate naming.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>