spi: add support for all spi modes with soft spi

The spi bitbanging driver did not implement all spi modes properly. Add
code to support all spi modes, honoring soft_spi_set_mode() and
defaulting to spi mode 0. Previously, CPHA was implemented inversely
(defaulting to CPHA=1) and CPOL=1 was hardcoded.

Signed-off-by: Johannes Holland <johannes.holland@infineon.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>