bcma: Allow selection of this driver when COMPILE_TEST=y

This allows us to increase compile-test coverage without having to build
a kernel for MIPS.  That's particularly interesting for subsystem
maintainers that want to test as many drivers as possible in a single
build.

We also add a dependency on HAS_IOMEM in BCMA_HOST_SOC to make sure the
driver is not selected when the arch does not implement IO accessors.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>