ARM: at91: declare the at91rm9200 memory controller as a syscon

The at91rm9200 memory controller is not simply an SDRAM controller. It also
controls the EBI (External Bus Interface), the SMC (Static Memory Controller)and
the Burst Flash Controller.

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