dts: fix location of stm32-specific property

The max-erase-time property was introduced for the STM32 flash driver,
but it was inserted as an optional property in the generic
soc-nv-flash binding which is used by other SoCs.

Make it a required property in a new st,stm32-nv-flash binding
instead, since it is at present a vendor specific property.

Update the DTS files accordingly. Keep the existing "soc-nv-flash"
value in the compatible list in each case, so that DT_HAS_COMPAT(...
soc_nv_flash) tests on these nodes will still succeed, but put it
after a newly added "st,stm32-nv-flash" compatible, so that the
SoC-specific binding will be used as it is discovered first by the DT
tooling.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>