serial: sh-sci: Enable the driver on all ARM platforms

Renesas ARM platforms are transitioning from single-platform to
multi-platform kernels using the new ARCH_SHMOBILE_MULTI. Make the
driver available on all ARM platforms to enable it on both ARCH_SHMOBILE
and ARCH_SHMOBILE_MULTI, and increase build testing coverage with
COMPILE_TEST.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>