Add support for the Calao SBC35-A9G20 board

The Calao SBC35-A9G20 board is manufactured and sold by Calao Systems
<http://www.calao-systems.com>. It is built around an AT91SAM9G20 ARM SoC
running at 400MHz. It features an Ethernet port, an SPI RTC backed by an onboard
battery , an SD/MMC slot, a CompactFlash slot, 64Mo of SDRAM, 256Mo of NAND
flash, two USB host ports, and an USB device port. More informations can be
found at <http://www.calao-systems.com/articles.php?lng=en&pg=5936

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Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>