spi: mxc: fix sf probe when using mxc_spi

spi: mxc: fix sf probe when using mxc_spi

MXC SPI driver has a feature whereas a GPIO line can be used to force CS high
across multiple transactions. This is set up by embedding the GPIO information
in the CS value:

cs = (cs | gpio << 8)

This merge of cs and gpio data into one value breaks the sf probe command:
if the use of gpio is required, invoking "sf probe <cs>" will not work, because
the CS argument doesn't have the GPIO information in it. Instead, the user must
use "sf probe <cs | gpio << 8>". For example, if bank 2 gpio 30 is used to force
cs high on cs 0, bus 0, then instead of typing "sf probe 0" the user now must
type "sf probe 15872".

This is inconsistent with the description of the sf probe command, and forces
the user to be aware of implementaiton details.

Fix this by introducing a new board function: board_spi_cs_gpio(), which will
accept a naked CS value, and provide the driver with the relevant GPIO, if one
is necessary.

Cc: Eric Nelson <[email protected]>
Cc: Eric Benard <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Marek Vasut <[email protected]>
Reviewed-by: Marek Vasut <[email protected]>
Signed-off-by: Nikita Kiryanov <[email protected]>
Reviewed-by: Jagannadha Sutradharudu Teki <[email protected]>