MFC: r287013 (partial), r294616, r314097 (partial)

- Allow overriding the FDT slicer with a custom slicer.
- Teach the flashmap code about SPI flash.
- Allow different slicers for different flash types to be registered
with geom_flashmap(4) and teach it about MMC for slicing enhanced
user data area partitions. The FDT slicer still is the default for
CFI, NAND and SPI flash on FDT-enabled platforms.
- In addition to a device_t, also pass the name of the GEOM provider
in question to the slicers as a single device may provide more than
one provider.
- Build a geom_flashmap.ko.
- Use MODULE_VERSION() so other modules can depend on geom_flashmap(4).
- Remove redundant/superfluous GEOM routines that either do nothing
or provide/just call default GEOM (slice) functionality.
- Trim/adjust includes

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a new geom class which allows to divide NAND Flash chip
into partitions.

Partitions are created based on data in dts file which are
extracted and interpreted by slicer.

Obtained from: Semihalf
Supported by: FreeBSD Foundation, Juniper Networks