microblaze: Add missing PVR version codes

PVR version code was missing in the cpu_ver_lookup table for the following
versions: 8.50.b 8.50.c 9.2 9.3

This caused /proc/cpuinfo to display "CPU-Ver: Unknown" for these versions.

This was detected and the patch tested with MicroBlaze version 8.50.c.
The other codes were taken from the Xilinx MicroBlaze Processor Reference
Guides UG081 (v14.7) and UG984 (v2014.1).

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>