spi: davinci: add support for interrupt mode

Add support for SPI interrupt mode operation.

Define a per chip-select "io type" variable which
specifies if the transfers on this chip-select should
happen in interrupt mode or polled mode.

Introduce a new function davinci_spi_process_events()
to help consolidate the code between interrupt mode
processing and polled mode processing.

Signed-off-by: Brian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>