arm: mvebu: add DB-88F6820-AMC board

This board is a plug in card for Marvell's switch system development
kits. Form-factor aside it is similar to the DB-88F6820-GP with the
following differences.
- TCLK is 200MHz
- SPI1 is used
- No SATA
- No MMC
- NAND flash

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>