dt-bindings: display: verisilicon,dc: make resets non-optional

dt-bindings: display: verisilicon,dc: make resets non-optional

The only variant supported by the binding now, the DC8200 controller on
T-Head TH1520, requires three reset lines.

Add the reset properties to the required list to clarify this. The
in-tree th1520.dtsi file isn't affected by this change because the reset
lines are already populated there.

The driver still considers the resets optional, so if another variant
with no reset line appears, only the binding needs tweaking.

Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260702054804.164820-1-zhengxingda@iscas.ac.cn
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>