arm: mvebu: helios4: Reset uSOM onboard phy during board init

Similar to Clearfog rev 2.1, GPIO 19 also used to reset onboard ethernet
PHY.

This patch depend on
net: mvneta: Add GPIO configuration support
[URL: https://patchwork.ozlabs.org/patch/1007736/

]

Signed-off-by: Aditya Prayoga <aditya@kobol.io>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-By: Dennis Gilmore <dgilmore@redhat.com>
Reviewed-By: Dennis Gilmore <dgilmore@redhat.com>
Signed-off-by: Stefan Roese <sr@denx.de>