arch/tile: add hypervisor-based character driver for SPI flash ROM

The first version of this patch proposed an arch/tile/drivers/ directory,
but the consensus was that this was probably a poor choice for a place to
group Tilera-specific drivers, and that in any case grouping by platform
was discouraged, and grouping by function was preferred.

This version of the patch addresses various issues raised in the
community, primarily the absence of sysfs integration.  The sysfs
integration now handles passing information on sector size, page size,
and total partition size to userspace as well.  In addition, we now
use a single "struct cdev" to manage all the partition minor devices,
and dynamically discover the correct number of partitions from the
hypervisor rather than using a module_param with a default value.

This driver has no particular "peer" drivers it can be grouped with.
It is sort of like an MTD driver for SPI ROM, but it doesn't group well
with the other MTD devices since it relies on hypervisor vi...