ARM: dts: turris-omnia: configure LED[0] pin function to link/activity

The marvell PHY driver changes the LED[0] pin function to "On - 1000
Mbps Link, Off - Else".

Turris Omnia expects that the function is "On - Link, Blink - Activity,
Off - No link".

Use the `marvell,reg-init` DT property to change the function.

In the future, once netdev trigger will support HW offloading, we will
be able to have this configured via the combination of PHY driver and
leds-turris-omnia driver.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>