ARM: at91/pm: Remove show_reset_status function

Both the reset and poweroff drivers are now implementing what the
show_reset_status function used to do. Hence, we can remove this rather hackish
function that prevents us from doing further cleanup.

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>