kernel: Add missing includes mtdsplit_*.c

This fixes the following compile problem with kernel 4.9 on lantiq:
drivers/mtd/mtdsplit/mtdsplit_uimage.c:244:34: error: array type has incomplete element type 'struct of_device_id'
 static const struct of_device_id mtdsplit_uimage_of_match_table[] = {
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/mtdsplit/mtdsplit_uimage.c:245:4: error: field name not in record or union initializer
  { .compatible = "denx,uimage" },

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>