drivers: flash: stm32h7: support flash controller driver on M4

Currently flash controller driver builds and runs only on M7.
This patch supports enablement of the driver on M4 CPUs. The
main issue in using the driver on M4 is that LL_GetFlashSize()
to read the flash size works only on M7 as the internal register
is not accessible from M4. So to use the driver on M4, add a dt
property, bank2-flash-size, to configure flash size of bank2.
this will allow gradual support of flash controller driver
on M4 of all supported STMH7 boards by defining the above
dt property and testing it. Currently this is verified only
on STM32H747i-disco board.

Signed-off-by: Murali Karicheri <murali.karicheri@sandc.com>