apm821xx: set mib-poll-interval on emac0 attached ar83xx switch

Commit "generic: ar8216: add mib_poll_interval switch attribute" sets
mib-poll-interval as disabled by default (was set to 2s), so it makes
switch LEDs trigger disfunctional on devices which don't have
mib-poll-interval set.

So this patch sets mib-poll-interval to 500ms on devices which have
ar83xx switch connected to emac0, as the same value was set for built in
switches in 443fc9ac35 ("ath79: use ar8216 for builtin switch").

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/apm821xx/dts/meraki-mx60.dts

@@ -100,6 +100,8 @@
 		phy0: ethernet-phy@0 {
 			compatible = "ethernet-phy-id004d.d034";
 			reg = <0>;
+			qca,mib-poll-interval = <500>;
+
 			qca,ar8327-initvals = <
 				0x0010 0x40000000
 				0x0624 0x007f7f7f

target/linux/apm821xx/dts/netgear-wndr4700.dts

@@ -293,6 +293,8 @@
 		phy0: ethernet-phy@0 {
 			device_type = "ethernet-phy";
 			reg = <0>;
+			qca,mib-poll-interval = <500>;
+
 			qca,ar8327-initvals = <
 				0x0010 0x40000000
 				0x0624 0x007f7f7f