clk: Assert prepare_lock in clk_core_get_boundaries

clk: Assert prepare_lock in clk_core_get_boundaries

This function iterates the clk consumer list on clk_core so it must be
called under prepare_lock. This is already done by all callers but add a
lockdep assert to check anyway.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Link: https://lkml.kernel.org/r/29453ee8e820457d87a8faf9d496390e59c6826f.1562073871.git.leonard.crestez@nxp.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>