MAINTAINERS: change mailing list address for Altera UART drivers

The nios2-dev list has been moved to the RocketBoards infrastructure, so
adjust the address accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>