AT91 watchdog timer controller driver

[Andrew Lunn <andrew at lunn dot ch>]

As i said in my last email, the AT91 system controller has a different
watchdog timer than the older devices. This patch contains a driver
for this new hardware.

A couple of points to note:

1) The watchdog timer is driven by the slow clock of the AT91. This is
an LC oscillator on the chip. The accuracy of this oscillator is
dependent on silicon process at manufacturing time and the
temperature. I found that my development board a watchdog timeout of
1000ms actually goes off at around 550ms at room temperature. So be
generous with your timeouts. This also means that one of the test case
fails because the timer goes off too early.

2) The mode register on the watchdog is read/write once. So once the
watchdog has been configured it cannot be changed without a reset. It
is possible if you are using Redboot it disables the watchdog and so
the application cannot then use it. Similarly a JTAG debugger could
disable it. I've put in an assertion check to detect this condition,
so if the watchdog does not appear to be working enable INFRA_DEBUG
and see if the assertion goes off.

        Andrew

Attachment: devs.watchdog.arm.at91wdtc.diff
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