doc: bindings: soft-spi: update documentation to match the code

Linux bindings have been introduced in the code (removing the U-Boot
specific ones) without documentation update. Compatible string has
changed, as well as the four GPIO properties. Reflect this by updating
the soft-spi.txt documentation.

Fixes: 102412c4

 ("dm: spi: soft_spi: switch to use linux compatible string")
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>