ot1200: fix sf detection

Commit 155fa9af changed the way
to define a GPIO line, which can be used to force CS high
across multiple transactions. In order to fix sf detection
change board code to make use of board_spi_cs_gpio(..).

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>