ARM: dts: stm32: fix gpio-keys node on STM32 MCU boards

Fix following warning observed with "make dtbs_check W=1" command.
It concerns f429 eval and disco boards, f769 disco board.

Warning (unit_address_vs_reg): /gpio_keys/button@0: node has a unit name,
but no reg or ranges property

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>