dm: Add a power sequencing uclass

Some devices need special sequences to be used when starting up. Add a
uclass for this. Drivers can be added to provide specific features as
needed.

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

include/dm/uclass-id.h

@@ -51,6 +51,7 @@ enum uclass_id {
 	UCLASS_PINCTRL,		/* Pinctrl (pin muxing/configuration) device */
 	UCLASS_PINCONFIG,	/* Pin configuration node device */
 	UCLASS_PMIC,		/* PMIC I/O device */
+	UCLASS_PWRSEQ,		/* Power sequence device */
 	UCLASS_REGULATOR,	/* Regulator device */
 	UCLASS_RESET,		/* Reset device */
 	UCLASS_REMOTEPROC,	/* Remote Processor device */