devicetree: serial: Document msm_serial bindings

Let's fix up the msm serial device bindings so that it's clearer
what hardware is supported. Instead of using hsuart (for high
speed uart) let's use uartdm because that matches the actual name
of the hardware. Also, let's add the version information in case
we need to differentiate between different versions of the
hardware in the future. Finally, lets specify that clocks are
required (the clock bindings didn't exist when the original
binding was written) and also specify dma bindings just in case
we want to use it in software. We split the binding into two
files to make it clearer what's required and not required.

Cc: David Brown <davidb@codeaurora.org>
Cc: <devicetree@vger.kernel.org>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>