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/debug-s5l8702.c

@@ -58,6 +58,8 @@ bool dbg_hw_info(void)
         if(state == 0)
         {
             _DEBUG_PRINTF("CPU:");
+            _DEBUG_PRINTF("speed: %d MHz", ((CLKCON0 & 1) ?
+                                CPUFREQ_NORMAL : CPUFREQ_MAX) / 1000000);
             _DEBUG_PRINTF("current_tick: %d", (unsigned int)current_tick);
             line++;