drm/sun4i: Move rate variables to long long

Our clock rate variables are getting pretty close to the LONG_MAX / ULONG_MAX
limit, especially since we will start doing arithmetic on it. Move those
types to unsigned long long to be sure we don't overflow their type.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com> # tested on pinebook
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/022c3b850413edd6afbca20062f100971de2f5af.1551191081.git-series.maxime.ripard@bootlin.com