hw/arm: Add basic power management to raspi.

hw/arm: Add basic power management to raspi.

This is just enough to make reboot and poweroff work. Works for
linux, u-boot, and the arm trusted firmware. Not tested, but should
work for plan9, and bare-metal/hobby OSes, since they seem to generally
do what linux does for reset.

The watchdog timer functionality is not yet implemented.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/64
Signed-off-by: Nolan Leake <nolan@sigbus.net>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210625210209.1870217-1-nolan@sigbus.net
[PMM: tweaked commit title; fixed region size to 0x200;
 moved header file to include/]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>