clk: sunxi-ng: allow set parent clock (PLL_CPUX) for CPUX clock on H3

The CPUX clock, which is the main clock of the ARM core on Allwinner H3,
can be adjusted by changing the frequency of the PLL_CPUX clock.

Allowing setting parent clock for the CPUX clock, thus the PLL_CPUX
clock can be adjusted when adjusting the CPUX clock.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Fixes: 0577e485 ("clk: sunxi-ng: Add H3 clocks")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>