spi: orion: cosmetics - alias long direct_access variables

This change increases the source code readability.
Instead of using `spi->child[cs].direct_access.XXX` use `dir_acc->XXX`.
Instead of using `orion_spi->child[cs].direct_access.vaddr` use `vaddr`.

Signed-off-by: Kosta Zertsekel <zertsekel@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>