time: rename time source option from FRC to "high-resolution timer"

time: rename time source option from FRC to "high-resolution timer"

libc time function now rely on esp_timer_get_time as the source of
high-resolution time, rather than FRC1 timer. Internally, on the ESP32
esp_timer implementation uses FRC2 timer.

- Change help text and labels in Kconfig to use "high-resolution timer"
  instead of FRC1. Keep existing Kconfig option name to be backwards
  compatible.
- Change references to "FRC1" in the source code to "FRC".