spi: kconfig: add soft spi Kconfig entry

Add the Kconfig entry for SOFT_SPI which uses gpio to simulate the
SPI signals. We use it for accessing 74x164 on some i.MX boards.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Jagan Teki <jagan@openedev.com>
Cc: Stefano Babic <sbabic@denx.de>