Add syscon power and reset control device driver

Add syscon power and reset control device driver

This device driver supports both syscon-power and syscon-reset devices,
as specified in [1] and [2]. These provide a very simple interface for
power and reset control, and among other things are used by QEMU's virt
machine on RISC-V. A separate commit will enable this on RISC-V, as that
requires adding a RISC-V-specific riscv_syscon akin to r327936's
aw_syscon.

[1] https://www.kernel.org/doc/Documentation/devicetree/bindings/power/reset/syscon-poweroff.txt
[2] https://www.kernel.org/doc/Documentation/devicetree/bindings/power/reset/syscon-reboot.txt

Reviewed by:	brooks (mentor), jhb (mentor)
Approved by:	brooks (mentor), jhb (mentor)
Obtained from:	CheriBSD
Differential Revision:	https://reviews.freebsd.org/D25724