fs/fatfs: Add CONFIG_FS_FATFS_MIN_SS

The Kconfig option allows to set minimum expected sector size
to be supported by FAT fs driver.
When this value differs from CONFIG_FS_FATFS_MAX_SS the driver
will query device for actual sector size, expecting different
sector sizes for different device. When CONFIG_FS_FATFS_MIN_SS
and CONFIG_FS_FATFS_MAX_SS are the same, then there is slight
reduction if FAT driver size, as the query logic is removed
and CONFIG_FS_FATFS_MAX_SS is used for all devices.

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