Documentation: arm/Marvell: clarify Armada SoCs that match 78xx0 pattern

New users of Marvell SoCs will potentially be confused by the MVEBU
SoCs that match the 78xx0 pattern and thus which defconfig and mach-*
directory to be looking at.  Add a bit of clarification to README for
this.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>