This commit enables soc-flash support in stm32f746g_disco only for
flash test. Using overlays, it adds soc-flash storage partition and
deletes qspi-flash storage partition for flash test. Both flash test and
spi_flash application has been tested on stm32f746g_disco platform.
Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>