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 virtualization
to handle many of the irritating aspects of flash ROM management: sector
awareness, background read for sub-sector writes, bit examination to
determine whether a sector erase needs to be issued, etc.  It is in fact
more like an EEPROM driver, but the hypervisor virtualization does require
a "flush" command if you wish to commit a sector write prior to writing
to a different sector, and this is sufficiently different from generic
I2C/SPI EEPROMs that as a result it doesn't group well with them either.

The simple character device is already in use by a range of Tilera
SPI ROM management tools, as well as by customers.  In addition, using
the simple character device actually simplifies the userspace tools,
since they don't need to manage sector erase, background read, etc.
This both simplifies the code (since we can uniformly manage plain files
and the SPI ROM) as well as makes the user code portable to non-Linux
platforms that don't offer the same MTD ioctls.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>

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