clk: Add clk_min/max_rate entries in debugfs

Add two files to expose min/max clk rates as determined by
clk_core_get_boundaries, taking all consumer requests into account.

This information does not appear to be otherwise exposed to userspace.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Link: https://lkml.kernel.org/r/68e96af2df96512300604d797ade2088d7e6e496.1562073871.git.leonard.crestez@nxp.com
[sboyd@kernel.org: Drop if statements for JSON printing]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>