clk: rockchip: add npll to source of sclk_gpu

The possible sources for the rk3288-gpu-clock also include the npll,
making it the same list of sources as for uart0.

This patch make a common source for uart0 pll src and sclk_gpu,
so that gpu can get its clock from npll.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>