m68k: improve m68knommu MAINTAINERS entry

Improve the information in the m68knommu maintainers entry. This
should aid in making it clearer what parts of the m68k architecture
code can go via the m68knommu git tree.

Specifically the entry now lists the relevant git tree where m68knommu
patches are promoted through. It also spells out that the coldfire
sub-architecture, and with it the directory of arch/m68k/coldfire, as
being supported via this tree.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>