ARM: efm32: properly namespace i2c location property

Wolfram Sang pointed out during review of an efm32-i2c driver that the
property to specify the set of pins has a too general name. As several
other efm32 peripherals also have a similar register bit field, add an
"efm32" namespace.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/boot/dts/efm32gg-dk3750.dts

@@ -26,7 +26,7 @@ adc@40002000 {
 		};
 
 		i2c@4000a000 {
-			location = <3>;
+			efm32,location = <3>;
 			status = "ok";
 
 			temp@48 {