net: phy: add default ARCH_BCM_IPROC for MDIO_BCM_IPROC

Add default MDIO_BCM_IPROC Kconfig setting such that it is default
on for IPROC family of devices.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>