mips64el in installation manual

Hello,

For the coming d-i alpha release, I have added the mips64el architecture
to the installation manual. I have basically copied mipsel bits, since
AIUI mips64el is merely the 64bit version of mipsel. It would be worth
proofreading it. Basically it boils down to:

svn co svn://svn.debian.org/svn/d-i/trunk/manual
cd manual
dpkg-checkbuilddeps
cd build
./buildone mips64el en pdf

and look at build.out/install.en.pdf

There is notably the Section 2.1.2. CPU, Main Boards, and Video Support
which I merely copied from mipsel.  I guessed the comment about Malta
was the same for mips64el.

You can also run

grep -r mips64el en/

to see the sections which I have enabled.

Thanks,
Samuel