dt-bindings: net: Describe BCM7445 switch reset property

The BCM7445/BCM7278 built-in Ethernet switch have an optional reset line
to the SoC's reset controller, describe the 'resets' and 'reset-names'
properties as optional.

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