spi: orion: fix comment to mention MVEBU

MVEBU chips (Armada XP, Armada 370 and others) are supported by this
driver. Mention this in the help text to make more obvious what is
already specified in the dependencies of this symbol.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>