Boot crashes due to global async pending list

[Mark Brown <broonie@opensource.wolfsonmicro.com>]

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Booting current -next causes crashes during regulator_bulk_enable() in
the async code (the function uses an async domain to run multiple calls
to regulator_enable() in parallel, synchronising the domain to finish
them).  Reverting commit 9fdb04 (async: replace list of active domains
with global list of pending items) causes the problem to go away.

I've not got any sensible diagnostics at the minute, the crash was in
async_entry_function() in one of the list_del_init() calls suggesting a
locking issue but everything looks in order there.  Will try to
investigate properly when I get time, I appreciate that this isn't an
awesome bug report.  If there's any diagnostics that'd be helpful please
let me know.

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