parisc: Replace regular spinlock with spin_trylock on panic path

The panic notifiers' callbacks execute in an atomic context, with
interrupts/preemption disabled, and all CPUs not running the panic
function are off, so it's very dangerous to wait on a regular
spinlock, there's a risk of deadlock.

Refactor the panic notifier of parisc/power driver to make use
of spin_trylock - for that, we've added a second version of the
soft-power function. Also, some comments were reorganized and
trailing white spaces, useless header inclusion and blank lines
were removed.

Cc: "James E.J. Bottomley" <[email protected]>
Cc: Jeroen Roovers <[email protected]>
Acked-by: Helge Deller <[email protected]> # parisc
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Signed-off-by: Helge Deller <[email protected]>

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