kernel: add seil-fw mtdsplit driver for IIJ SEIL devices

kernel: add seil-fw mtdsplit driver for IIJ SEIL devices

This mtdsplit parser driver parses firmware partition on Internet
Initiative Japan Inc. (IIJ) SEIL series devices.

Structure of header:

  0x0 - 0x7 : Identifier            (hex)
  0x8 - 0x57: Copyright             (ascii)
 0x58 - 0x5b: Data CRC              (hex)
 0x5c - 0x5f: Image Format Version  (hex)
 0x60 - 0x63: Image Major Version   (hex)
 0x64 - 0x67: Image Minor Version   (hex)
 0x68 - 0x87: Image Release Version (ascii)
 0x88 - 0x8b: Xor value for Data?   (hex)
 0x8c - 0x8f: Data Length           (hex)

Properties:

- compatible      : "iij,seil-firmware"
- iij,seil-id     : ID of SEIL firmware for the device (8 bytes)
  - examples:
    - SA-W2       : <0x5345494c 0x32303135> ("SEIL2015")
    - SEIL/X1     : <0x5345494c 0x2F582020> ("SEIL/X  ")
- iij,bootdev-name: boot device name assigned to the partition
                    (optional)

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>