Board-rk29sdk.c:vmac need mac_ref clock to be 50MHZ.so modify pll clock to 300MHZ to fit it

arch/arm/mach-rk29/board-rk29sdk.c

@@ -1893,7 +1893,7 @@ static void __init machine_rk29_mapio(void)
 	rk29_map_common_io();
 	rk29_setup_early_printk();
 	rk29_sram_init();
-	rk29_clock_init(periph_pll_288mhz);
+	rk29_clock_init(periph_pll_300mhz);
 	rk29_iomux_init();
 }