drivers: led_strip: Add support for external SPI CS on APA102 LED strips

Some boards may include an APA102 LED on an existing SPI bus. With
additional circuitry, chip select is supported where the APA102
lacks a built in CS pin.

This patch allows CS to be defined in the devicetree (through .dts
files) and utilised in the apa102.c driver if it's available. Preserving
old behaviour if no chip select is defined.

Signed-off-by: Roman Vaughan <nzsmartie@gmail.com>