ARM: dts: bcm283x: Align ETH_CLK GPIO line name

The GPIO line name ETHCLK is not aligned with the other signals like
WIFI_CLK. Recently this has been fixed in the vendor tree, so upstream
this change.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>