dt-bindings: stm32: document stm32mp135f-dk board

Add new entry for stm32mp135f-dk board.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rob Herring <robh@kernel.org>