clk: Mark clk_core_evict_parent_cache_subtree() 'target' const

Clarify that the 'target' clk isn't being modified, instead it's being
searched for. Mark it const so the function can't modify it.

Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20220217220554.2711696-3-sboyd@kernel.org