samples/fs/fat_fs: Add support for nrf52840dk_nrf52840

The commits adds support for nrf52840dk_nrf52840 with two additional
configurations:
 - with FAT FS on internal SoC flash
 - with FAT FS on QSPI connected MX25

In case of SoC configuration there is re-configuration of internal
partitions done, doe to need for at least of 64kiB for FAT FS.
The QSPI configuration only defines additional partition on
external device.

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