phy: phy-ocelot-serdes: add ability to be used in a non-syscon configuration

The phy-ocelot-serdes module has exclusively been used in a syscon setup,
from an internal CPU. The addition of external control of ocelot switches
via an existing MFD implementation means that syscon is no longer the only
interface that phy-ocelot-serdes will see.

In the MFD configuration, an IORESOURCE_REG resource will exist for the
device. Utilize this resource to be able to function in both syscon and
non-syscon configurations.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>