spi: s3c64xx: Allow driver to build if COMPILE_TEST is enabled

The driver only has runtime but no build time dependency with PLAT_SAMSUNG
|| ARCH_EXYNOS so it can be built for testing purposes if the COMPILE_TEST
option is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>