ARM: at91/dt: fix at91 udc compatible strings

The at91rm9200, at91sam9260, at91sam9261 and at91sam9263 SoCs have slightly
different UDC IPs.
Those differences were previously handled with cpu_is_at91xx macro which
are about to be dropped for multi-platform support, thus we need to
change compatible strings.

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