powerpc/pseries/vas: sysfs interface to export capabilities

The hypervisor provides the available VAS GZIP capabilities such
as default or QoS window type and the target available credits in
each type. This patch creates sysfs entries and exports the target,
used and the available credits for each feature.

This interface can be used by the user space to determine the credits
usage or to set the target credits in the case of QoS type (for DLPAR).

/sys/devices/vas/vas0/gzip/default_capabilities (default GZIP capabilities)
	nr_total_credits /* Total credits available. Can be
			 /* changed with DLPAR operation */
	nr_used_credits  /* Used credits */

/sys/devices/vas/vas0/gzip/qos_capabilities (QoS GZIP capabilities)
	nr_total_credits
	nr_used_credits

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/702d8b626ebfac2b52f4995eebeafe1c9a6fcb75.camel@linux.ibm.com