dm: Add a system reset uclass

It is common for system reset to be available at multiple levels in modern
hardware. For example, an SoC may provide a reset option, and a board may
provide its own reset for reasons of security or thoroughness. It is useful
to be able to model this hardware without hard-coding the behaviour in the
SoC or board. Also there is a distinction sometimes between resetting just
the CPU (leaving GPIO state alone) and resetting all the PMICs, just cutting
power.

To achieve this, add a simple system reset uclass. It allows multiple devices
to provide reset functionality and provides a way to walk through them,
requesting a particular reset type until is it provided.

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

include/dm/uclass-id.h

@@ -44,6 +44,7 @@ enum uclass_id {
 	UCLASS_PCI_GENERIC,	/* Generic PCI bus device */
 	UCLASS_PMIC,		/* PMIC I/O device */
 	UCLASS_REGULATOR,	/* Regulator device */
+	UCLASS_RESET,		/* Reset device */
 	UCLASS_RTC,		/* Real time clock device */
 	UCLASS_SERIAL,		/* Serial UART */
 	UCLASS_SPI,		/* SPI bus */