Convert pipe tasklet to threaded irq

Tasklets have long been deprecated as being too heavy on the system
by running in irq context - and this is not a performance critical
path. If a higher priority process wants to run, it must wait for
the tasklet to finish before doing so. A more suitable equivalent
is to converted to threaded irq instead and deal with the signaled
pipes in task context.

Change-Id: Ia8fc1b7b44712d8a677ead8bd607c94c2efc8e04
Link: https://lore.kernel.org/r/20210115002014.117528-1-dave@stgolabs.net


Signed-off-by: Roman Kiryanov <rkir@google.com>