MAINTAINERS: add the linuppc-dev list to the fsl-mc bus entry

MAINTAINERS: add the linuppc-dev list to the fsl-mc bus entry

As discussed in the thread linked below, the fsl-mc bus lacked a clear
maintenance path. Since Christophe accepted to take the fsl-mc bus
patches through his soc fsl subtree, add the linuxppc-dev mailing list
in the MAINTAINERS entry.

Link: https://lore.kernel.org/r/1d822960-85a7-42b3-88cf-9d3dbc75a831@csgroup.eu
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20250320120319.3520315-4-ioana.ciornei@nxp.com
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>