spi: orion: add LSB support

The orion spi driver currently only supports the normal (i.e. MSB) mode.
This patch adds LSB first mode.

Also correct the comment about supported SPI modes that was left over by
b15d5d70 ("spi/orion: Add SPI_CHPA and SPI_CPOL support to kirkwood
driver.").

Signed-off-by: Bastian Stender <bst@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>