santino:fec: Add external oscillator

On Garz & Fricke i.MX6QDL boards an external oscillator is being used for the Ethernet reference clock. Add the clock as fixed-clock and ste it in the FEC device tree.

The MUX and the GPR1[21] Register needs to be set accordingly. to achieve this we use the imx6q_1588_init() arch/arm/mach-imx/mach-imx6q.c initialization code. It matched the FEC clocks and checks whether the FEC reference clock is the same as the internal "enet_ref" clock. If it isn't the GPR1_ENET_CLK_SEL is cleared resulting in the external oscillator being used.

Ethernet seems to work but somehow I still get errors from the NetworkManager:

[  OK  ] Stopped Network Manager.
         Starting Network Manager...
[  OK  ] Started Login Service.
[FAILED] Failed to start Network Manager.
See 'systemctl status NetworkManager.service' for details.
[  OK  ] Started Hostname Service.
         Starting Network Manager Script Dispatcher Service...
[  OK  ] Stopped Network Manager.
         Starting Network Manager...
[  OK  ] Started Network Name Resolution.
[  OK  ] Reached target Host and Network Name Lookups.
         Starting Avahi mDNS/DNS-SD Stack...
[  OK  ] Started Network Manager Script Dispatcher Service.
[  OK  ] Started Avahi mDNS/DNS-SD Stack.
[FAILED] Failed to start Network Manager.
See 'systemctl status NetworkManager.service' for details.
[  OK  ] Reached target Network.
         Starting DNS forwarder and DHCP server...
[  OK  ] Stopped Network Manager.
         Starting Network Manager...
[  OK  ] Started DNS forwarder and DHCP server.
[FAILED] Failed to start Network Manager.
See 'systemctl status NetworkManager.service' for details.
[  OK  ] Reached target Multi-User System.
         Starting Update UTMP about System Runlevel Changes...
[  OK  ] Stopped Network Manager.
         Starting Network Manager...
[  OK  ] Started Update UTMP about System Runlevel Changes.
[FAILED] Failed to start Network Manager.
See 'systemctl status NetworkManager.service' for details.