clk: imx8: fix build warning

When build clk driver in spl, met the warning:
"
drivers/clk/imx/clk-imx8.c:21:25: warning: ‘imx8_clk_names’ defined but not used [-Wunused-variable]
 static struct imx8_clks imx8_clk_names[] = {
                         ^~~~~~~~~~~~~~
"

Fix with wrapping the array with CONFIG_CMD_CLK.

Signed-off-by: Peng Fan <peng.fan@nxp.com>