Probe HPET existence

QEMU does not provide a HPET block if it was configured with -no-hpet,
other machines SeaBIOS runs on may lack a HPET as well. Perform basic
checks the ID register for a reasonable vendor ID and a clock period
within the valid range, do not build the HPET table if that fails.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>