dt-bindings: Remove PicoXcell bindings

PicoXcell has had nothing but treewide cleanups for at least the last 8
years and no signs of activity. The most recent activity is a yocto vendor
kernel based on v3.0 in 2015.

Cc: linux-crypto@vger.kernel.org
Acked-by: Jamie Iles <jamie@jamieiles.com>
Link: https://lore.kernel.org/r/20201210200315.2965567-5-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>