net: ks8851: Add optional vdd_io regulator and reset gpio

Allow the ks8851 driver to enable an optional 1.8V vdd_io
regulator and assert the reset pin to the phy if a reset gpio is
present in device tree.

Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>