tty: serial: Restrict ST ASC driver to only build on !ARM when COMPILE_TEST is set.

The Kconfig for this option suggests it's only relevant on STi SoCs,
so add depends to have it not show up on archs that won't ever run it.

Signed-off-by: Dave Jones <davej@fedoraproject.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>