net/designware: add support of max-speed device tree property

This property allows to specify fastest connection mode supported by
the MAC (as opposed to features of the phy).

There are situations when phy may handle faster modes than the
MAC (or even it's particular implementation or even due to CPU being too
slow).

This property is a standard one in Linux kernel these days and some
boards do already use it in their device tree descriptions.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
cc: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>