MAINAINTERS: Corrected Broadcom Northstar2 entry

While moving the Northstar 2 DTS into a dedicated directory, the
corresponding MAINTAINERS file entry was not updated accordingly, fix
that.

Fixes: 63a913c1 ("arm64: dts: move ns2 into northstar2 directory")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>