ARM: at91: remove useless at91rm9200_dt_initialize()

at91rm9200_dt_initialize() is doing the same as at91_dt_initialize(), use that
one instead.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>