spi: spi-orion: enable the driver on ARCH_MVEBU platforms

The SPI controller managed by the spi-orion is used on the new ARM64
Marvell Armada 7K/8K SoCs. In order to allow this driver to be built
for this platform, we allow it to be selected for ARCH_MVEBU=y
configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>