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/kernel-s5l8702.c

@@ -39,13 +39,14 @@ void tick_start(unsigned int interval_in_ms)
 {
     int cycles = 10 * interval_in_ms;
     
-    /* configure timer for 10 kHz */
+    /* configure timer for 10 kHz (external source) */
     TBCMD = (1 << 1);   /* TB_CLR */
-    TBPRE = 337 - 1;    /* prescaler */
+    TBPRE = 75 - 1;     /* prescaler */         /* 12 MHz / 16 / 75 = 10 KHz */
     TBCON = (0 << 13) | /* TB_INT1_EN */
             (1 << 12) | /* TB_INT0_EN */
             (0 << 11) | /* TB_START */
             (2 << 8) |  /* TB_CS = PCLK / 16 */
+            (1 << 6) |  /* UNKNOWN bit */       /* external 12 MHz clock (?) */
             (0 << 4);   /* TB_MODE_SEL = interval mode */
     TBDATA0 = cycles;   /* set interval period */
     TBCMD = (1 << 0);   /* TB_EN */