arm: mvebu: disable unused Armada RTC on ReadyNAS 102, 104 and 2120

By default, armada-370-xp.dtsi file has internal RTC enabled.
NETGEAR ReadyNAS 102, 104 and 2120 all use an Intersil ISL12057
I2C RTC chip. The internal RTC not being disabled in the .dts
files of those devices result in the following useless first
line during boot:

[    4.500056] rtc-mv d0010300.rtc: internal RTC not ticking
[    4.505684] i2c /dev entries driver
[    4.513246] rtc-isl12057 0-0068: rtc core: registered rtc-isl12057 as rtc0

This patch marks Armada internal RTC as disabled in individual .dts
files of those devices.
Reported-by: TuxOholic <tuxoholic@hotmail.de>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/armada-370-netgear-rn102.dts

@@ -83,6 +83,12 @@ pcie@2,0 {
 		};
 
 		internal-regs {
+
+			/* RTC is provided by Intersil ISL12057 I2C RTC chip */
+			rtc@10300 {
+				status = "disabled";
+			};
+
 			serial@12000 {
 				status = "okay";
 			};

arch/arm/boot/dts/armada-370-netgear-rn104.dts

@@ -83,6 +83,12 @@ pcie@2,0 {
 		};
 
 		internal-regs {
+
+			/* RTC is provided by Intersil ISL12057 I2C RTC chip */
+			rtc@10300 {
+				status = "disabled";
+			};
+
 			serial@12000 {
 				status = "okay";
 			};

arch/arm/boot/dts/armada-xp-netgear-rn2120.dts

@@ -90,6 +90,12 @@ pcie@5,0 {
 		};
 
 		internal-regs {
+
+			/* RTC is provided by Intersil ISL12057 I2C RTC chip */
+			rtc@10300 {
+				status = "disabled";
+			};
+
 			/* Two rear eSATA ports */
 			sata@a0000 {
 				nr-ports = <2>;