ARM/dom0: Avoid using a variable length array in make_memory_node()

The reg[] array can have a maximum size of 8 in practice, so use the worst
case calculation rather than making it variable length.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>

xen/arch/arm/domain_build.c

@@ -539,9 +539,11 @@ static int __init make_memory_node(const struct domain *d,
     int res, i;
     int reg_size = addrcells + sizecells;
     int nr_cells = reg_size*kinfo->mem.nr_banks;
-    __be32 reg[nr_cells];
+    __be32 reg[NR_MEM_BANKS * 4 /* Worst case addrcells + sizecells */];
     __be32 *cells;
 
+    BUG_ON(nr_cells >= ARRAY_SIZE(reg));
+
     dt_dprintk("Create memory node (reg size %d, nr cells %d)\n",
                reg_size, nr_cells);