net: dsa: bcm_sf2: Add support for optional reset controller line

Grab an optional and exclusive reset controller line for the switch and
manage it during probe/remove functions accordingly. For 7278 devices we
change bcm_sf2_sw_rst() to use the reset controller line since the
WATCHDOG_CTRL register does not reset the switch contrary to stated
documentation.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>