Add the binding documentation of the mtk-pmic-keys for the MT6358 PMICs.

MT6358 is a little different since it used separate IRQs for the
release key (_r) event
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220121140323.4080640-3-mkorpershoek@baylibre.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>