serial: implement common uart post test

The current arch/driver specific UART posts basically boil down to setting
the UART to loop back mode, then reading and writing data.  If we ignore
the loop back part, the rest can be built upon the existing common serial
API.  So let's do just that.

First add a call back for serial drivers to implement loop back control.
Then write a post test that walks all of the serial drivers, puts them
into loop back mode, and verifies that reading/writing at all the diff
baud rates is OK.

If a serial driver doesn't support loop back mode (either it can't or
it hasn't done so yet), then skip it.  This should allow for people to
easily migrate to the new post test with existing serial drivers.

I haven't touched the few already existing uart post tests as I don't
the hardware or knowledge of converting them over.  So I've marked the
new test as weak which will allow the existing tests to override the
default until they are converted.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

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