serial: 8250: extend compile-test coverage

Allow more drivers to be compile tested more easily, for example, when
doing subsystem-wide changes.

Verified on X86_64 as well as arm, powerpc and m68k with minimal configs
in order to catch missing implicit build dependencies.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210715083011.18887-1-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>