[IMX8MM] clock: revert VIDEO_PLL_RATE for IMX8MM

In commit 3c60a494 ([i.MX8MP][DRIVER] configure display clk tree, 2024-07-23)
the VIDEO_PLL_RATE was adjusted for IMX8MP. Use #ifdef to select the correct value
for IMX8MM.

arch/arm/mach-imx/imx8m/clock_imx8mm.c

@@ -280,7 +280,11 @@ int intpll_configure(enum pll_clocks pll, ulong freq)
 	return 0;
 }
 
+#ifdef CONFIG_IMX8MP
 #define VIDEO_PLL_RATE 1039500000U
+#else
+#define VIDEO_PLL_RATE 594000000U
+#endif
 
 void mxs_set_lcdclk_type(u32 base_addr, u32 freq, enum lcdifv3_type type)
 {