net: zynq_gem: Use max-speed property from dt

Add support to use max-speed property from dt for
determining the supported speed. Use 1000Mbps as
default.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>