tty: serial: Limit msm_serial_hs driver to platforms that use it

The msm_serial_hs driver uses mach specific dma APIs. This is not
compatible with the multi-platform ARM effort. Let's only compile
this driver on MSM devices that are prepared to support it;
allowing the DT based MSM devices to enter the multi-platform ARM
build.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Olof Johansson <olof@lixom.net>