Add watchdog core about tillitis-key1 HOT 6 CLOSED

The https://github.com/tillitis/tillitis-key1/tree/watchdog branch contains a (hopefully) working implementation. It needs review and testing.

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When attempting to add support for the hardware watchdog in qemu I noticed that exposing TK1_MMIO_WATCHDOG_CURR_TIMER means we have to update it 18 million times per second. This slows down qemu quite a lot.

I propose we don't expose CURR_TIMER to software at all.

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qemu support in branch watchdog in https://github.com/tillitis/qemu

Still WIP. Timers and MMIO work and the watchdog() callback function is hit but we still can't reset the machine properly.

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Commit 01fab59 remove the CURR_TIMER from the API.

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An important question is if the watchdog improves the ability to perform warm-boot attacks?

By crafting an app that simply enables the watchdog after n cycles it allows an attacker to control when a device is being reset, and get the FPGA to configure itself without disconnecting it (and reconnecting it again) to the host. An attacker may be able to repeatedly try to perform a warm-boot attack and extract information. Possibly.

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Will not be integrated in the TKey1 device.

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