mtd: nand: export nand_get_flash_type function

`nand_get_flash_type()` allows identification of supported NAND flashs.
The function is useful in SPL (like mxs_nand_spl.c) to lookup for a NAND
flash (which does not support ONFi) instead of using nand_simple.c and
hard-coding all required NAND parameters.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>