fs: littlefs: add alignment arg to FS_LITTLEFS_DECLARE_CUSTOM_CONFIG macro

Add alignment parameter to FS_LITTLEFS_DECLARE_CUSTOM_CONFIG macro, it
speeds up read/write operation for SDMMC devices in case when we align
buffers on CONFIG_SDHC_BUFFER_ALIGNMENT, because we can avoid extra copy
of data from card bffer to read/prog buffer. See, how 'card_read_blocks'
function works.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>