spi: gpio: Advertise support for SPI_CS_HIGH

The spi-gpio driver already handles different chip select polarities,
but so far this was not advertised in master->mode_bits.

This patch fixes mmc_spi on top of spi_gpio, which is useful in some
testing scenarios.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Mark Brown <broonie@kernel.org>