xtensa: ISS: add host file-based simulated disk

Simdisk is a block device that maps to a file in the host file system.
It is usable for testing in the simulated environment, like xt-sim or
QEMU. Device binding to host file may be changed at runtime via proc
interface provided the device is not in use. Number of block devices
and initial binding to host files is controlled via kernel/module
parameters, with defaults specified in the kernel configuration.

Signed-off-by: Victor Prupis <vnp@tensilica.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>