ARM: at91: Remove rstc and shdwnc global base addresses

Now that there's no user left for the global variables holding the reset and
shutdown controllers base address, we can remove these variables and their
associated mapping function.

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>