spi: davinci: add support for multiple bus and chip select

Currently davinci spi driver supports only bus 0 cs 0.
This patch allows driver to support bus 1 and bus 2 with
configurable number of chip selects. Also defaults are
selected in a way to avoid regression on other platforms
that uses davinci spi driver and has only one spi bus.

Signed-off-by: Rex Chang <rchang@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>