MAINTAINERS: exclude other Socionext SoC DT files from ARM/UNIPHIER entry

There are several product lines in Socionext.  I maintain only
DT files prefixed with "uniphier".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>