powerpc/mpic: Don't open-code dcr_resource_start

Don't open-code the OpenFirmware "dcr-reg" property lookup trying to map
DCR resources.  This makes the code a bit easier to read.
Signed-off-by: NKyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/sysdev/mpic.c

@@ -319,11 +319,8 @@ static void _mpic_map_dcr(struct mpic *mpic, struct device_node *node,
 			  struct mpic_reg_bank *rb,
 			  unsigned int offset, unsigned int size)
 {
-	const u32 *dbasep;
-
-	dbasep = of_get_property(node, "dcr-reg", NULL);
-
-	rb->dhost = dcr_map(node, *dbasep + offset, size);
+	phys_addr_t phys_addr = dcr_resource_start(node, 0);
+	rb->dhost = dcr_map(mpic->node, phys_addr + offset, size);
 	BUG_ON(!DCR_MAP_OK(rb->dhost));
 }