RSIR is 0x1 (POR) if we woke from Deep Sleep.
This makes it hard to distinguish between real power-on and waking from
Deep Sleep, which is why the Backup RAM signature was introduced.
However, calling cpu_woke_from_backup() a second time will always return
true, as the signature will have been set up by early boot then.
Thus, clear the POR bit if the signature was already in place.
The result is:
RSIR == 0 -> woke from sleep
RSIR == 1 -> cold boot
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