ARM: mach-shmobile: mackerel: update defconfig

* Enable ARM_APPENDED_DTB

  Typically the bootloader of a mackerel board does not support DT
  so this option is useful

* Add "rw" to command line

  This appears to be necessary for a successful NFS-root boot

* Remove memchunk from kernel command line,
  it is not used outside of arch/sh

* Move command line to dts

  This brings us one small step closer to sharing defconfig
  between mackerel and other boards

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>