doc: update timer HLD doc after modularization

Replace rdstc() and get_tsc_khz() with their architectural agnostic
counterparts cpu_ticks() and cpu_tickrate().

Tracked-On: #5920
Signed-off-by: Yi Liang <yi.liang@intel.com>

hypervisor/include/arch/x86/asm/tsc.h

@@ -34,6 +34,9 @@ uint32_t get_tsc_khz(void);
 /**
  * @brief Calibrate Time Stamp Counter (TSC) frequency.
  *
+ * @remark Generic time related routines, e.g., cpu_tickrate(), us_to_ticks(),
+ * udelay(), etc., relies on this function being called earlier during system initialization.
+ *
  * @return None
  */
 void calibrate_tsc(void);