dm: Add support for generic system controllers (syscon)

Many SoCs have a number of system controllers which are dealt with as a
group by a single driver. It is a pain to have to add lots of compatible
strings and/or separate drivers for each. Instead we can identify the
controllers by a number and request the address of the one we want.

Add a simple implementation of this which can be used by SoC driver code.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/uclass-id.h

@@ -46,6 +46,7 @@ enum uclass_id {
 	UCLASS_SPI,		/* SPI bus */
 	UCLASS_SPI_FLASH,	/* SPI flash */
 	UCLASS_SPI_GENERIC,	/* Generic SPI flash target */
+	UCLASS_SYSCON,		/* System configuration device */
 	UCLASS_THERMAL,		/* Thermal sensor */
 	UCLASS_USB,		/* USB bus */
 	UCLASS_USB_DEV_GENERIC,	/* USB generic device */