drivers: flash: stm32wba6x add flash driver support

-The stm32wba6x has Dual Bank memory. Change the flash driver
 to support this OPTion given by presence of the
 DUAL_BANK bit (21) in the FLASH_OPTR register.

-Flash erase with 2 banks: Add the control of the BKER
 bit of the FLASH_NSCR1 to select BANK1 or 2 of
 the internal flash depending
 on the page number >127 for BANK2

Signed-off-by: Francois Ramu <francois.ramu@st.com>