spi: lantiq-ssc: activate under COMPILE_TEST

This driver should compile on all platforms, activate it under compile
test. The Lantiq specific parts are under ifdef and should be removed
when Lantiq platform supports common clock framework.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Mark Brown <broonie@kernel.org>