drivers: led_strip: apa102: add multi instances support

This allows for multiple apa102 strips on multiple spi ports.

For example:

```
&spi1 {
	led_strip_0: apa102@0 {
		compatible = "apa,apa102";
		reg = <0>;
		spi-max-frequency = <5250000>;
	};
};
&spi2 {
	led_strip_1: apa102@0 {
		compatible = "apa,apa102";
		reg = <0>;
		spi-max-frequency = <5250000>;
	};
};
```

Signed-off-by: Francois Gervais <francoisgervais@gmail.com>