ARM: stm32: Supply a DTS file for the STM32F469 Discovery board

It's pretty similar to the STM32F429, but there are some
subtle changes required to boot successfully.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>