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>