drivers: flash_stm32h7: Fix REAL_FLASH_SIZE_KB size

For devices other than the stm32h745/747/755/757 that use
dual-core cortex-M4/cortex-M7, do not multiply flash
controller size by 2. Single-core stm32h7x devices define
their flash-controllers dtsi as a single area, not two banks.
Use the presence of the bank2-flash-size prop to determine if
this is a dual-core stm3h7x or not.

Signed-off-by: Rory Piper <rory.piper@signal-fire.com>