samples/soc_flash_nrf: Fix soc_flash_nrf doc and printfs

The commit fixes soc_flash_nrf sample documentation where incorrect
partition names has been used and removes information of
CONFIG_TRUSTED_EXECUTION_NONSECURE affecting which partition is used
as this is no longer true.

The printf message reporting start of sample has been modified
to print "Nordic nRF5 Flash Sample" instead of
"Nordic nRF5 Flash Testing", which is more accurate for code
residing in samples.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>