mx31pdk: Change maintainer

Currently I don't have access to a mx31pdk board.

Magnus was the original maintainer of the board and accepted to take back
this role.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Magnus Lilja <lilja.magnus@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>