mailbox: arm_mhu: Constify amba_id table

'struct amba_id' table is not modified so can be changed to const for
more safety.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/arm_mhu.c

···
 	return 0;
 }
 
-static struct amba_id mhu_ids[] = {
+static const struct amba_id mhu_ids[] = {
 	{
 		.id	= 0x1bb098,
 		.mask	= 0xffffff,