usb: gadget: at91-udc: fix irq and iomem resource retrieval

When using dt resources retrieval (interrupts and reg properties) there is
no predefined order for these resources in the platform dev resource
table. Also don't expect the number of resource to be always 2.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Acked-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable <stable@vger.kernel.org> # 3.4
Signed-off-by: Felipe Balbi <balbi@ti.com>