ARM: at91/dt: declare matrix node as a syscon device

There is no specific driver handling the AHB matrix, this is a simple syscon
device. the matrix is needed by several other drivers including the USB on some
SoCs (at91sam9261 for instance).
Without this definition, the USB will not work on these SoCs.

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