dt-bindings: wkup-m3-ipc: Add firmware-name property

Document that the firmware-name property can be used to indicate a file
contains I2C sequences for PMIC voltage scaling during deep sleep.

Based on previous work by Russ Dill.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
[dfustini: split from driver patch and convert to json-schema]
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220426200741.712842-2-dfustini@baylibre.com