clk: pxa: Fix const discarding warning

A recent change to mark parent names as const missed this struct
member so we get warnings like:

drivers/clk/pxa/clk-pxa25x.c:122:2: warning: initialization
discards 'const' qualifier from pointer target type

Fix it.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/pxa/clk-pxa.h

@@ -72,7 +72,7 @@ struct desc_clk_cken {
 	const char *name;
 	const char *dev_id;
 	const char *con_id;
-	const char **parent_names;
+	const char * const *parent_names;
 	struct clk_fixed_factor lp;
 	struct clk_fixed_factor hp;
 	struct clk_gate gate;