stmmac: socfpga: Provide dt node to config ptp clk source.

Provides an options to use the ptp clock routed from the Altera FPGA
fabric. Instead of the defalt eosc1 clock connected to the ARM HPS core.
This setting affects all emacs in the core as the ptp clock is common.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>