mvebu: Add rudimental MV78230 support

This adds basic support for Marvell's MV78230 SoC which belongs to the
Armada XP series.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>