powerpc/mm: Replace open coded CONTEXT_BITS value

To clarify the meaning for future readers, replace the open coded
19 with CONTEXT_BITS
Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mmu_context_hash64.c

@@ -34,7 +34,7 @@ static DEFINE_IDA(mmu_context_ida);
  * Each segment contains 2^28 bytes.  Each context maps 2^44 bytes,
  * so we can support 2^19-1 contexts (19 == 35 + 28 - 44).
  */
-#define MAX_CONTEXT	((1UL << 19) - 1)
+#define MAX_CONTEXT	((1UL << CONTEXT_BITS) - 1)
 
 int __init_new_context(void)
 {