ARM: at91: setup: Switch to pr_fmt

Most of the printed messages are using the "AT91:" prefix in the setup.c file,
but not all of them. Moreover, those who add it hardcode it directly in the
message, while the pr_fmt macro makes it easier for us to support such a case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>