overlays: sc16is75x: Add generic SPI overlay #3625

Job	Run time
build (bcm2835, arm, bcm2835_defconfig, kernel)	9m 26s
build (arm64, arm64, defconfig, kernel8)	37m 23s
build (bcmrpi, arm, bcmrpi_defconfig, kernel)	31m 49s
build (bcm2709, arm, bcm2709_defconfig, kernel7)	31m 35s
build (bcm2711, arm64, bcm2711_defconfig, kernel8)	41m 45s
build (bcm2711_rt, arm64, bcm2711_rt_defconfig, kernel8_rt)	42m 51s
build (bcm2712, arm64, bcm2712_defconfig, kernel_2712)	41m 53s
	3h 56m 42s

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