dt-bindings: firmware: thead,th1520: Add support for firmware node

dt-bindings: firmware: thead,th1520: Add support for firmware node

The kernel communicates with the E902 core through the mailbox
transport using AON firmware protocol. Add dt-bindings to document it
the dt node.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
Acked-by: Drew Fustini <drew@pdp7.com>
Link: https://lore.kernel.org/r/20250311171900.1549916-2-m.wilczynski@samsung.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>