clk: lochnagar: Add a determine_rate hook

The lochnagar clocks implement a mux with a set_parent hook, but
doesn't provide a determine_rate implementation.

This is a bit odd, since set_parent() is there to, as its name implies,
change the parent of a clock. However, the most likely candidates to
trigger that parent change are either the assigned-clock-parents device
tree property or a call to clk_set_rate(), with determine_rate()
figuring out which parent is the best suited for a given rate.

The other trigger would be a call to clk_set_parent(), but it's far less
used, and it doesn't look like there's any obvious user for that clock.

Since there's no upstream device tree using that driver, it's a bit hard
to tell if it uses the assigned-clock properties. The binding and its
example uses them though, so it's likely that the author intent was to
force the parent through the device tree and prevent any reparenting but
through an explicit call to clk_set_parent().

This case is equivalent to setting the determine_rate implementation to
clk_hw_determine_rate_no_reparent(). Indeed, if no determine_rate
implementation is provided, clk_round_rate() (through
clk_core_round_rate_nolock()) will call itself on the parent if
CLK_SET_RATE_PARENT is set, and will not change the clock rate
otherwise.

Cc: Charles Keepax <ckeepax@opensource.cirrus.com>
Cc: Richard Fitzgerald <rf@opensource.cirrus.com>
Cc: patches@opensource.cirrus.com
Tested-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/clk/clk-lochnagar.c

@@ -209,6 +209,7 @@ static u8 lochnagar_clk_get_parent(struct clk_hw *hw)
 static const struct clk_ops lochnagar_clk_ops = {
 	.prepare = lochnagar_clk_prepare,
 	.unprepare = lochnagar_clk_unprepare,
+	.determine_rate = clk_hw_determine_rate_no_reparent,
 	.set_parent = lochnagar_clk_set_parent,
 	.get_parent = lochnagar_clk_get_parent,
 };