clk: imx: make mux parent strings const

As the commit 2893c379 ("clk: make strings in parent name arrays
const"), let's make the parent strings const, otherwise we may meet
the following warning when compiling:

drivers/clk/imx/clk-imx7ulp.c: In function 'imx7ulp_clocks_init':
drivers/clk/imx/clk-imx7ulp.c:73:35: warning: passing argument 5 of
	'imx_clk_mux_flags' discards 'const' qualifier from pointer target type

  clks[IMX7ULP_CLK_APLL_PRE_SEL] = imx_clk_mux_flags("apll_pre_sel", base + 0x508, 0,
	1, pll_pre_sels, ARRAY_SIZE(pll_pre_sels), CLK_SET_PARENT_GATE);
                                   ^
In file included from drivers/clk/imx/clk-imx7ulp.c:23:0:
drivers/clk/imx/clk.h:200:27: note: expected 'const char **' but argument is
 of type 'const char * const*'
...

Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>