powerpc/powernv: Add OPAL flash device driver

Firmware needed by petitboot, for example, GPU firmware, can be installed to
a partition in the flash filesystem. This driver exposes the full flash
given by the device tree, letting the user manage firmware, etc, from
FreeBSD.

To use the partitions provided by the flash module, the fdt_slicer module is
needed, but the module isn't needed for raw access, so there's no direct
dependency link in here.

MFC after: 2 weeks