target-ppc: Use #define instead of opencoding SLB valid bit

Use SLB_ESID_V instead of (1 << 27) in the code

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

target-ppc/mmu_helper.c

@@ -2061,7 +2061,7 @@ void helper_store_sr(CPUPPCState *env, target_ulong srnum, target_ulong value)
         /* ESID = srnum */
         rb |= ((uint32_t)srnum & 0xf) << 28;
         /* Set the valid bit */
-        rb |= 1 << 27;
+        rb |= SLB_ESID_V;
         /* Index = ESID */
         rb |= (uint32_t)srnum;