arm: mvebu: clearfog: reset uSOM onboard 1512 phy

Use GPIO19 which is wired to the uSOM phy reset signal in order to reset
the uSOM's 1512 Gigabit Ethernet phy.

This GPIO is valid on ClearFog rev 2.1 and newer.

Taken from SolidRun's specialised u-boot, see
https://github.com/SolidRun/u-boot-armada38x/commit/f906e3df172e07ac82cdd87b278d7896949262ea



Signed-off-by: Patrick Wildt <patrick@blueri.se>
Signed-off-by: Stefan Roese <sr@denx.de>