clk: sunxi-ng: add CLK_SET_RATE_PARENT flag to H3 GPU clock

The GPU clock on H3 has only one parent, PLL-GPU, and the PLL is only
the parent of the GPU clock. The GPU clock can be tweaked by tweaking
the PLL-GPU clock.

Add CLK_SET_RATE_PARENT flag to allow tweaking the GPU clock via
tweaking PLL-CPU.

Fixes: 0577e485 ("clk: sunxi-ng: Add H3 clocks")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>