spi: bitbang: Implement support for MOSI idle state configuration

Some SPI peripherals may require strict MOSI line state when the controller
is not clocking out data. Implement support for MOSI idle state
configuration (low or high) by setting the data output line level on
controller setup and after transfers. Bitbang operations now call
controller specific set_mosi_idle() callback to set MOSI to its idle state.
The MOSI line is kept at its idle state if no tx buffer is provided.

Acked-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/de61a600b56ed9cb714d5ea87afa88948e70041e.1720810545.git.marcelo.schmitt@analog.com


Signed-off-by: Mark Brown <broonie@kernel.org>