[flash] Teach mx25l SPI flash driver to interact with fdt_slicer and geom_flashmap

This teaches the mx25l driver (sys/dev/flash/mx25l.c) to interact with
sys/dev/fdt/fdt_slicer.c and sys/geom/geom_flashmap.c.

This allows systems with SPI flash to benefit from the possibility to define
flash 'slices' via FDT, just the same way that it's currently possible for
CFI and NAND flashes.

Tested:

• Carambola 2, AR9331 + SPI NOR flash

PR: kern/206227
Submitted by: Stanislav Galabov <sgalabov@gmail.com>