kirkwood: make MPP arrays static const

This saves stack and code memory for local copy, and
consumes initialized data memory. For 22 of the 29
kirkwood-based boards, this results in a global saving
of about 30 bytes. For 7 of them, it results in an
increase of 6 to 14 bytes.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>