samples: usb: mass: add disk description to overlays

Add flash disk description to overlays. Specify custom flashdisk
partition for fat fs api test to match the Kconfig values, because
native posix storage partition is too small for FatFS filesystem.

Co-authored-by: Johann Fischer <johann.fischer@nordicsemi.no>
Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>