ARM: shmobile: kzm9d: Conditionally select SMSC_PHY

The kzm9d board uses has an SMSC911X ethernet controller which uses an
SMSC phy. Select SMSC_PHY for kzm9d if SMSC911X is enabled to make use of the
SMSC-specific phy driver rather than relying on the generic phy driver.

This only covers the case of multiplatform kzm9d
as there is currently no Kconfig node for non-multiplatform kzm9d.
One could be added if desired.

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