spi: orion: Add direct access mode

This patch adds support for the direct access mode to the Orion SPI
driver which is used on the Marvell Armada based SoCs. In this direct
mode, all data written to (or read from) a specifically mapped MBus
window (linked to one SPI chip-select on one of the SPI controllers)
will be transferred directly to the SPI bus. Without the need to control
the SPI registers in between. This can improve the SPI transfer rate in
such cases.

Both, direct-read and -write mode are supported. But only the write
mode has been tested. This mode especially benefits from the SPI direct
mode, as the data bytes are written head-to-head to the SPI bus,
without any additional addresses.

One use-case for this direct write mode is, programming a FPGA bitstream
image into the FPGA connected to the SPI bus at maximum speed.

This mode is described in chapter "22.5.2 Direct Write to SPI" in the
Marvell Armada XP Functional Spec Datasheet.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>