PCI: mvebu: Add help string for CONFIG_PCI_MVEBU option

There is no description for CONFIG_PCI_MVEBU option. Add it.

Link: https://lore.kernel.org/r/20220104153529.31647-6-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>