README: update MIPS related informations

Amend section 'Directory Hierarchy' for current MIPS directory.
Describe config options for MIPS.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Acked-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>