pnv/xive2: Enable VST NVG and NVC index compression

pnv/xive2: Enable VST NVG and NVC index compression

Enable NVG and NVC VST tables for index compression which indicates the number
of bits the address is shifted to the right for the table accesses.
The compression values are defined as:
   0000 - No compression
   0001 - 1 bit shift
   0010 - 2 bit shift
   ....
   1000 - 8 bit shift
   1001-1111 - No compression

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Kowal <kowal@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>