imx: power-domain: Enable refcounting on imx8mp

Prevent enabling/disabling multiple times the same power domain to avoid
breakages due to the same power domains being referenced several times
by different device nodes.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
diff --git a/drivers/power/domain/imx8m-power-domain.c b/drivers/power/domain/imx8m-power-domain.c
index e54ba5d..b44aae7 100644
--- a/drivers/power/domain/imx8m-power-domain.c
+++ b/drivers/power/domain/imx8m-power-domain.c
@@ -506,8 +506,12 @@
 static int imx8m_power_domain_probe(struct udevice *dev)
 {
 	struct imx8m_power_domain_plat *pdata = dev_get_plat(dev);
+	struct power_domain_plat *plat = dev_get_uclass_plat(dev);
 	int ret;
 
+	/* Every subdomain has its own device node */
+	plat->subdomains = 1;
+
 	/* Nothing to do for non-"power-domain" driver instances. */
 	if (!strstr(dev->name, "power-domain"))
 		return 0;