kern/riscv/efi/init: Use time register in grub_efi_get_time_ms()

kern/riscv/efi/init: Use time register in grub_efi_get_time_ms()

The cycle register is not guaranteed to count at constant frequency.
If it is counting at all depends on the state the performance monitoring
unit. Use the time register to measure time.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>