ARM: 5782/1: at91: support for eco920

CONFIG_MACH_ECO920 is enabled in at91rm9200dk_defconfig.  The name is
wrong, but this is better than adding another defconfig or don't get
compile coverage at all.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>