tests: drivers: flash: change integration_platforms

drivers.flash.nrf_qspi_nor and drivers.flash.soc_flash_nrf:
keep nrf52840dk_nrf52840 as integration_platform

drivers.flash.default:
Use mimxrt1060_evk instead as integration_platform,
this is the only platform allowed.

drivers.flash.stm32:
Add a bunch of boards as integration_platforms for
this test configuration.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>