[timer] Formalise the timer API

We now have two implementations for the timer API: one using the
time-of-day counter at 40:70 and one using RDTSC.  Both make use of
timer2_udelay().

src/config/defaults/pcbios.h

@@ -9,6 +9,7 @@
 
 #define IOAPI_X86
 #define PCIAPI_PCBIOS
+#define TIMER_PCBIOS
 #define CONSOLE_PCBIOS
 
 #endif /* CONFIG_DEFAULTS_PCBIOS_H */