spi: dw: Handle any number of gpiod CS lines

Even when configured to use only gpiod CS lines, the DW SPI controller
still expects a bit to be set in the SER register, otherwise transfers
stall. For the csgpiod case, nominate bit 0 for the job.

See: https://github.com/raspberrypi/linux/issues/6159



Signed-off-by: Phil Elwell <phil@raspberrypi.com>