AT91: reset routine cleanup, remove not needed icache flush

Generalize assembler reset routine to allow use on several at91sam9 chips.
This patch replace double definitions of SDRAM controller registers and RSTC
registers with use of classical header files.

For this rework, we remove the not needed icache flush as it is already
done in the calling function: arm_machine_restart().

Rename at91sam9g20_reset.S to generalize to several chips.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>