clk: Add clk_get_rate_range

With the recent introduction of clock drivers that will force their
clock rate to either the minimum or maximum boundaries, it becomes
harder for clock users to discover either boundary of their clock.

Indeed, the best way to do that previously was to call clk_round_rate()
on either 0 or ULONG_MAX and count on the driver to clamp the rate to
the current boundary, but that won't work anymore.

Since any other alternative (calling clk_set_rate_range() and looking at
the returned value, calling clk_round_rate() still, or just doing
nothing) depends on how the driver will behaves, we actually are
punching a hole through the abstraction provided by the clock framework.

In order to avoid any abstraction violation, let's create a bunch of
accessors that will return the current minimum and maximum for a given
clock.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>