spidev: Support to dynamically add spidev to SPI bus

Adding parameters to spidev module, to allow it to be
added dynamically to the SPI bus. This allows a test
application to insmod it whenever needed and test the
SPI core.

Change-Id: Ifdb7b0c7a67f58d46ebcdc1032f0a654b6c53080
Signed-off-by: Harini Jayaraman <harinij@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>