imx: imx8mq: add init_nand_clk

Add init_nand_clk to enable gpmi nand clock. Since i.MX8MQ not use CCF,
so we still use legacy mode to configure the clock.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/include/asm/arch-imx8m/clock_imx8mq.h b/arch/arm/include/asm/arch-imx8m/clock_imx8mq.h
index 9fa9eb2..38a6f59 100644
--- a/arch/arm/include/asm/arch-imx8m/clock_imx8mq.h
+++ b/arch/arm/include/asm/arch-imx8m/clock_imx8mq.h
@@ -421,4 +421,6 @@
 	FRAC_PLL_OUT_1000M,
 	FRAC_PLL_OUT_1600M,
 };
+
+void init_nand_clk(void);
 #endif