avoid potentially uninitialised variable that likely won't happen.

GCC 12 can't tell that hardware / platform design means it won't happen.

branches: 1.2.2;
- sunxi_ccu_lcdxch1_set_rate(): enable parent before calling
sunxi_ccu_div_set_rate(). If it's not enabled get_rate() will return 0 and
sunxi_ccu_div_set_rate() will fail.
- rework sunxi_ccu_lcdxch0_set_rate() to actually really select the best
parent.

Add a round_rate() callback for the sunxi clock domain.
Add a sunxi_ccu_display.c file with helpers for setting up display engine
clocks.
for fractional clocks, rename frac_en to div_en, I got the logic inverted.
Adjust tcon0-ch0, tcon0-ch1, tcon1-ch0 and tcon1-ch1 definitions to
automatically select a parent. tcon0 hardcoded to pll3 and tcon1 to pll7.
Define a round_rate() callback for these clocks, as well as fractional clocks.
Hardcode debe clocks parent to pll5.