dsa: mv88e6xxx.c: Hardware reset the chip if available

The device tree binding now allows a gpio to be specified which is
attached to the switch chips reset line. If it is defined, perform
a hardware reset on the switch during setup.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>