MAINTAINERS: Change POWERPC MPC85XX maintainer to Marek Behún

MAINTAINERS: Change POWERPC MPC85XX maintainer to Marek Behún

After a discussion with Tom Rini, we've agreed that I am going to take
over custodianship of the MPC85XX platform, since it seems other people
do not have necessary interest or time and getting things done over
there takes too long.

Since I am only working on one MPC85XX board, Turris 1.x, and do not
have time to do thorough reviews of patches for this entire platform
(other than those concerning Turris 1.x board), for other boards I will
only run patches through CI and checkpatch, and then send them via PR
upwards to Tom.

Signed-off-by: Marek Behún <kabel@kernel.org>
Acked-by: Tom Rini <trini@konsulko.com>