manifest: move mips prebuilts into notdefault

  There aren't any MIPS devices regularly build on
  CM trees, and not syncing them by default reduces
  a significant transfer bandwidth and storage
  space for home builders.

Change-Id: I61798c66defa0e30d617bec1546e55880d43a445
Signed-off-by: Arnav Gupta <championswimmer@gmail.com>