dm: spi: soft_spi: switch to use linux compatible string

1. Support compatible string "spi-gpio" which is used by Linux
   Linux use different bindings, so use UBOOT_COMPAT and
   LINUX_COMPAT to differentiate them.
2. Introduce SPI_MASTER_NO_RX and SPI_MASTER_NO_TX to handle
   no rx or no tx case.
3. Tested on i.MX6 UltraLite board with 74LV595 spi-gpio chip.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>