dts: stm32h7: flash-controller: Add default STM32 as compatible

Add "st,stm32-flash-controller" as compatible for STM32H7 so that
what is defined for STM32 in general  is also defined for STM32H7.
Already most of the other STM32 versions have this addition.

Also removed the specific STM32H7 flag check in
/flash/driver/Kconfig.stm32.

Signed-off-by: Stefan Petersen <spe@ciellt.se>