MAINTAINERS: socfpga: update the git repo for SoCFPGA

The git tree at rocketboards.org is going away. Update the entry to reflect
the address of the new location. Also add an entry for all the socfpga_*
dts files.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>