mtd: nand: Consolidate nand spl loaders implementation

nand_spl_load_image implementation was copied over into three
different drivers and now with nand_spl_read_block used for
ubispl situation gets even worse. For now use least intrusive
solution and #include the same implementation to nand drivers.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Tested-by: Pau Pajuelo <ppajuel@gmail.com>