net: mdio: mscc-miim: Set back the optional resource.

In the blamed commit, the second memory resource was not considered
anymore as optional. On some platforms like sparx5 the second resource
is optional. So add it back as optional and restore the comment that
says so.

Fixes: a27a7628 ("net: mdio: mscc-miim: convert to a regmap implementation")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>