ARM: at91: Export at91_matrix_base

at91_matrix_* macro's are used by at91_udc usb gadget driver,
which can be built as module, therefore we need to export the
variable containing matrix base address.

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>