uart: add resume method

[Andriy Gapon]

on 23/12/2012 23:53 Garrett Cooper said the following:
> On Sun, Dec 23, 2012 at 5:36 AM, Andriy Gapon <avg at freebsd.org> wrote:
>>
>> Guys,
>>
>> do you think that the following change is useful/needed?
>>
>> I needed it with UART emulated in qemu, but I have no experience with real hardware.
> 
>     I assume that you suspended the emulator and the UART didn't come
> back to life?

No, I executed acpiconf -s 3 in the guest and then system_wakeup in the qemu
monitoring console.

-- 
Andriy Gapon