dm: pci: Update the PCI read_config() method to const dev *

At present this method uses a non-const udevice pointer, but the call
should not modify the device. Use a const pointer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/pci/pci_mpc85xx.c b/drivers/pci/pci_mpc85xx.c
index e58ab60..8dff68d 100644
--- a/drivers/pci/pci_mpc85xx.c
+++ b/drivers/pci/pci_mpc85xx.c
@@ -15,7 +15,7 @@
 	void __iomem		*cfg_data;
 };
 
-static int mpc85xx_pci_dm_read_config(struct udevice *dev, pci_dev_t bdf,
+static int mpc85xx_pci_dm_read_config(const struct udevice *dev, pci_dev_t bdf,
 				      uint offset, ulong *value,
 				      enum pci_size_t size)
 {