MAINTAINERS: update sparc maintainer

Reflect the current situation where David Miller
is the sparc maintainer.

I have tried to contact Bill on following adresses:

    wli@holomorphy.com
    wlirwin@us.ibm.com

with no success and Bill has not been active on the
sparclinux mailing list for a long time.

As sparc and sparc64 are unified I unified the two entries
in the MAINTAINERS file too.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>