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