davinci: Move serial platform_device into SoC-specific files

Currently, there is one set of platform_device and platform_data
structures for all DaVinci SoCs.  The differences in the data
between the various SoCs is handled by davinci_serial_init()
by checking the SoC type.  However, as new SoCs appear, this
routine will become more & more cluttered.

To clean up the routine and make it easier to add support for new
SoCs, move the platform_device and platform_data structures into the
SoC-specific code and use the SoC infrastructure to provide access
to the data.

In the process, fix a bug where the wrong irq is used for uart2
of the dm646x.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>