staging: mt7621-spi: add mt7621 support

NeilBrown:
    The code will fail with a warning if asked to transfer
    more than 32 bytes at a time.  So used max_transfer_size
    interface to tell users about this.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>