phy: enable compile-testing for the Lantiq PHY drivers

Unconditionally include the lantiq subdirectory in the phy Makefile.

All drivers in there have their dependencies maintained. One of these
(optional) dependencies is COMPILE_TEST, however this can only be
evaluated when Kconfig scans the lantiq subdirectory.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>