net: stmmac: socfgpa: Ensure emac bit set in System Manger for PTP

When using the PTP fpga to hps clock source for the stmmac module
the appropriate bit in the System Manager FPGA Interface Group register
needs to be set. This is not set by the bootloader setup  when the
HPS emac pins are being for this emac module.

This allows the PTP clock to be sourced from the FPGA and also connects
the PTP pps and ext trig signals to the stmmac PTP hardware.

Patch proposed by Phil Collins.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>