OMAP2+: voltage: keep track of powerdomains in each voltagedomain

When a powerdomain is registered and it has an associated voltage domain,
add the powerdomain to the voltagedomain using voltdm_add_pwrdm().

Also add voltagedomain iterator helper functions to iterate over all
registered voltagedomains and all powerdomains associated with a
voltagedomain.

Modeled after a similar relationship between clockdomains and powerdomains.

Signed-off-by: Kevin Hilman <khilman@ti.com>
diff --git a/arch/arm/mach-omap2/powerdomain.c b/arch/arm/mach-omap2/powerdomain.c
index 984457d..5164d58 100644
--- a/arch/arm/mach-omap2/powerdomain.c
+++ b/arch/arm/mach-omap2/powerdomain.c
@@ -99,6 +99,8 @@
 		return -EINVAL;
 	}
 	pwrdm->voltdm.ptr = voltdm;
+	INIT_LIST_HEAD(&pwrdm->voltdm_node);
+	voltdm_add_pwrdm(voltdm, pwrdm);
 
 	list_add(&pwrdm->node, &pwrdm_list);