tests/drivers: disco_l475_iot1: Enabling soc-flash support in overlays.

This commit enables soc-flash support in disco_l475_iot1
only for flash test. Using overlays, it adds soc-flash storage
partition and deletes qspi-flash storage parition for
flash test. Both flash test and spi_flash application
has been tested on disco_l475_iot1 platform.

Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>