powerpc-tunes: Introduce bigendian tune

powerpc-tunes: Introduce bigendian tune

This helps in defining LE tunes and at the same time specifies
endianness on compiler cmdline clearly, clang e.g. defaults to
little-endian always, so unless specified with -mbig-endian won't
compile the code right

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>