imx8m: add init_nand_clk

Add init_nand_clk to enable gpmi nand clock. Since i.MX8M not use CCF,
so we still use legacy mode to configure the clock.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>

arch/arm/include/asm/arch-imx8m/clock.h

@@ -256,6 +256,7 @@ u32 imx_get_fecclk(void);
 u32 imx_get_uartclk(void);
 int clock_init(void);
 void init_clk_usdhc(u32 index);
+void init_nand_clk(void);
 void init_uart_clk(u32 index);
 void init_usb_clk(void);
 void init_wdog_clk(void);