ARM: dts: stm32: Add gpio-keys for stm32mp157c-ed1-scmi-u-boot

Add 2 gpio-keys :
  _ button-user-1 for stm32prog mode activation.
  _ button-user-2 for fastboot mode activation.

Remove proprietary st,fastboot-gpios and st,stm32prog-gpios.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>