net: stmmac: implement the SIOCGHWTSTAMP ioctl

This patch adds support for the SIOCGHWTSTAMP ioctl which enables user
processes to read the current hwtstamp_config settings
non-destructively.

Signed-off-by: Artem Panfilov <panfilov.artyom@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>