MAINTAINERS: fix arch/arm/mach-at91/include/mach/at_hdmac.h

This file was moved to include/linux/platform_data/dma-atmel.h by commit
7cdc39ee

 ("ARM: at91: move platform_data definitions").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>