net: dsa: b53: Implement software reset for 58xx devices

Implement the correct software reset sequence for 58xx devices by
setting all 3 reset bits and polling for the SW_RST bit to clear itself
without a given timeout. We cannot use is58xx() here because that would
also include the 7445/7278 Starfighter 2 which have their own driver
doing the reset earlier on due to the HW specific integration.

Fixes: 991a36bb

 ("net: dsa: b53: Add support for BCM585xx/586xx/88312 integrated switch")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>