mmc: add the new linux-mmc mailing list to MAINTAINERS

There are a number of individual MMC drivers listed in MAINTAINERS.  I
didn't modify those records.  Perhaps I should have.

Cc: <linux-mmc@vger.kernel.org>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Nicolas Pitre <nico@cam.org>
Cc: Pierre Ossman <drzeus@drzeus.cx>
Cc: Pavel Pisa <ppisa@pikron.com>
Cc: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Sascha Sommer <saschasommer@freenet.de>
Cc: Ian Molton <ian@mnementh.co.uk>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>