AT91: Fix AT91SAM9G20 reset as per the errata in the data sheet

If the SDRAM is not cleanly shutdown before reset it can be left driving
the bus, which then stops the bootloader booting from NAND.

Signed-off-by: Peter Horton <phorton@bitbox.co.uk>
[nicolas.ferre@atmel.com: change file header line order]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>