ARM: sunxi: Remove reset code from the platform

Now that reset is handled either by the watchdog driver for the sun4i, sun5i
and sun7i, and by a driver of its own for sun6i, we can remove it from the
platform code.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>