MIPS: Expand help text to list generic defconfigs

Expand the MIPS Makefile help text to list generic board names, generic
defconfigs, and legacy defconfigs which have been converted to generic
and are still usable.

Here's a snippet of the new "make ARCH=mips help" output:
  ...
  If you are targeting a system supported by generic kernels you may
  configure the kernel for a given architecture target like so:

  {micro32,32,64}{r1,r2,r6}{el,}_defconfig <BOARDS="list of boards">

  Where BOARDS is some subset of the following:
    boston
    ni169445
    ranchu
    sead-3
    xilfpga

  Specifically the following generic default configurations are
  supported:

  32r1_defconfig           - Build generic kernel for MIPS32 r1
  32r1el_defconfig         - Build generic kernel for MIPS32 r1 little endian
  32r2_defconfig           - Build generic kernel for MIPS32 r2
  32r2el_defconfig         - Build generic kernel for MIPS32 r2 little endian
  32r6_defconfig           - Build generic kernel for MIPS32 r6
  32r6el_defconfig         - Build generic kernel for MIPS32 r6 little endian
  64r1_defconfig           - Build generic kernel for MIPS64 r1
  64r1el_defconfig         - Build generic kernel for MIPS64 r1 little endian
  64r2_defconfig           - Build generic kernel for MIPS64 r2
  64r2el_defconfig         - Build generic kernel for MIPS64 r2 little endian
  64r6_defconfig           - Build generic kernel for MIPS64 r6
  64r6el_defconfig         - Build generic kernel for MIPS64 r6 little endian
  micro32r2_defconfig      - Build generic kernel for microMIPS32 r2
  micro32r2el_defconfig    - Build generic kernel for microMIPS32 r2 little endian

  The following legacy default configurations have been converted to
  generic and can still be used:

  sead3_defconfig          - Build 32r2el_defconfig BOARDS=sead-3
  sead3micro_defconfig     - Build micro32r2el_defconfig BOARDS=sead-3
  xilfpga_defconfig        - Build 32r2el_defconfig BOARDS=xilfpga
  ...

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Matt Redfearn <matt.redfearn@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kbuild@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/18598/