MIPS: generic: Support MIPS Boston development boards

MIPS: generic: Support MIPS Boston development boards

Add support for the MIPS Boston development board to generic kernels,
which essentially amounts to:

  - Adding the device tree source for the MIPS Boston board.

  - Adding a Kconfig fragment which enables the appropriate drivers for
    the MIPS Boston board.

With these changes in place generic kernels will support the board by
default, and kernels with only the drivers needed for Boston enabled can
be configured by setting BOARDS=boston during configuration. For
example:

  $ make ARCH=mips 64r6el_defconfig BOARDS=boston

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16485/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>