at91: add support for RSIs EWS board

This adds support for the RSI EWS machine (at91rm9200-based),
machine type 1609.

Includes fixes as suggested by Uwe Kleine-König and
cleanups as suggested by Russell and R.Schwebel.

Signed-off-by: Josef Holzmayr <holzmayr@rsi-elektrotechnik.de>
[nicolas.ferre@atmel.com: simplify board file header, move to at91_init_irq_default]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>