net: phy: allow PHY drivers to implement their own software reset

As pointed out by Shaohui, most 10G PHYs out there have a non-standard
compliant software reset sequence, eventually something much more
complex than just toggling the BMCR_RESET bit. Allow PHY driver to
implement their own soft_reset() callback to deal with that. If no
callback is provided, call into genphy_soft_reset() which makes sure the
existing behavior is kept intact.

Reported-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>