patch - at91 serial drivers assumed realtime response for DSR routines

[Øyvind Harboe]

The at91 serial routines are broken in that they assume a very low
latency for the DSR handler.

This became a problem when I added a 16x5x serial port to my system,
which also spends a fair amount of time in the DSR handler.

My solution was to modify the work that was done to add hardware
buffering support to the at91 serial routines, but remove the rejected
modifications done to the interrupt routines. 

http://sources.redhat.com/ml/ecos-discuss/2002-01/msg00205.html

Although the ISR interrupt latency is still an issue, this should be a
step forward:

- it would bring the hardware buffering code under the scruteny of the
eCos community
- since the code is now in "at91 speak", a bit of the tedium has now
been removed if someone decides to do the "fast interrupt" support.

Testing done so far:

- I've tested that it works for UART0. I've got a maimed board where the
UART1 is physically disconnected, so I can't easily test this right now.
- The interoperability between AT91 uarts and 16x5x seem to be just fine
now.

Øyvind

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