Documentation: devicetree: amlogic: Document P20x and ODROID-C2 boards

Add compatible strings for Amlogic S905/GXBB based boards: Hardkernel
ODROID-C2, Amlogic P200 and Amlogic P201.

Cc: devicetree@vger.kernel.org
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>