Add VM_KMEM_SIZE_SCALE parameter set to 2 (50%) for all ARM platforms.

VM_KMEM_SIZE_SCALE specifies which fraction of the available physical
memory, after deduction of the kernel itself and other early statically
allocated memory, can be used for the kmem_map.  The kmem_map provides
for all UMA/malloc allocations in KVM space.

Previously ARM was using a fixed kmem_map size of (12*1024*1024) = 12MB
without regard to effectively available memory.  This is too small for
recent ARM SoC with more than 128MB of RAM.

For reference a description of others related kmem_map parameters:

 VM_KMEM_SIZE		default start size of kmem_map if SCALE is
			not defined
 VM_KMEM_SIZE_MIN	hard floor on the kmem_map size
 VM_KMEM_SIZE_MAX	hard ceiling on the kmem_map size
 VM_KMEM_SIZE_SCALE	fraction of the available real memory to
			be used for the kmem_map, limited by the
			MIN and MAX parameters.

Tested by:	ian
MFC after:	1 week

sys/arm/include/vmparam.h

@@ -134,12 +134,15 @@
 #endif
 
 #define VM_MAX_KERNEL_ADDRESS	0xffffffff
+
 /*
  * Virtual size (bytes) for various kernel submaps.
  */
-
 #ifndef VM_KMEM_SIZE
-#define VM_KMEM_SIZE            (12*1024*1024)
+#define VM_KMEM_SIZE		(12*1024*1024)
+#endif
+#ifndef VM_KMEM_SIZE_SCALE
+#define VM_KMEM_SIZE_SCALE	(2)
 #endif
 
 #define MAXTSIZ 	(16*1024*1024)