ARM: shmobile: mackerel: Conditionally select SMSC_PHY

The mackerel board uses has an SMSC911X ethernet controller which uses an
SMSC phy. Select SMSC_PHY for mackerel if SMSC911X is enabled to make use of the
SMSC-specific phy driver rather than relying on the generic phy driver.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>