Classic/6G: CPU frequency/voltage scaling

This patch implements HAVE_ADJUSTABLE_CPU_FREQ, it modifies the
following parameters when CPU is unboosted:

- s5l8702 voltage is decreased: 1.200V -> 1.050V
- CPU frequency is divided by 4: 216MHz -> 54MHz
- AHB frequency is divided by 2: 108MHz -> 54MHz

Change-Id: I2285b83efb7e1567864ac288f2d4ba55f058f7c5

firmware/target/arm/s5l8702/ipod6g/power-ipod6g.c

@@ -40,6 +40,9 @@ void power_off(void)
 void power_init(void)
 {
     idepowered = false;
+
+    /* DOWN1CTL: CPU DVM step time = 30us (default: no DVM) */
+    pmu_write(0x20, 2);
 }
 
 void ide_power_enable(bool on)