msm_serial: Add devicetree support

Add devicetree support to the msm_serial driver.  Clocks are still
queried by direct name from the driver until device tree clock support
is implemented.

Change-Id: Ia6b2ddfcf1e5dc3bd25dd502662f971202e6d56f
Signed-off-by: David Brown <davidb@codeaurora.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>