powerpc64: Set n_slbs = 32 for POWER9

Summary:
POWER9 also contains 32 slbs entries as explained by the POWER9 User Manual:

 "For HPT translation, the POWER9 core contains a unified (combined for both
   instruction and data), 32-entry, fully-associative SLB per thread"

Submitted by:	Breno Leitao
Differential Revision: https://reviews.freebsd.org/D15128

sys/powerpc/aim/aim_machdep.c

@@ -212,6 +212,7 @@ aim_early_init(vm_offset_t fdt, vm_offset_t toc, vm_offset_t ofentry, void *mdp,
 		case IBMPOWER7PLUS:
 		case IBMPOWER8:
 		case IBMPOWER8E:
+		case IBMPOWER9:
 			/* XXX: get from ibm,slb-size in device tree */
 			n_slbs = 32;
 			break;