ARM: at91: PIT: Use consistent exit path in probe

The current probe code is, whenever it fails to retrieve a resource, either
panicing, silently returning, or returning and displaying an error.

Make the code consistent by always panicing whenever it fails to grab any
resource.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>