jz4760: Disable dynamic clocking entirely.

Back off to 480MHz [max] clock, bus/mem clock of 120MHz.

576 is unstable on at least one unit, and 528 still glitches.

Change-Id: I020e48532524e739f3bfa42bed570381ccd34959

firmware/target/mips/ingenic_jz47xx/system-jz4760.c

@@ -451,7 +451,7 @@ static void pll0_init(unsigned int freq)
      *  DIV should be one of [1, 2, 3, 4, 6, 8]
      */
     const int div[2][6] = { { 1, 2, 2, 2, 2, 2 },
-			    { 1, 6, 6, 6, 6, 6 } };
+			    { 1, 4, 4, 4, 4, 4 } };
     const int n2FR[9] = {
         0, 0, 1, 2, 3, 0, 4, 0, 5
     };