net: dm: fec: Support phy-reset-post-delay property

As per Linux kernel DT binding doc:
- phy-reset-post-delay : Post reset delay in milliseconds. If present then
  a delay of phy-reset-post-delay milliseconds will be observed after the
  phy-reset-gpios has been toggled. Can be omitted thus no delay is
  observed. Delay is in range of 1ms to 1000ms. Other delays are invalid.

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@netmodule.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Lukasz Majewski <lukma@denx.de>