maintainers.rst: separate maintainership of uniphier and synquacer

Socionext has multiple product lines.  The UniPhier is not the only
platform any more.  Correct the maintainership.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

maintainers.rst

@@ -160,7 +160,7 @@ Masahiro Yamada (yamada.masahiro@socionext.com, `masahir0y`_)
 Files:
 
 - docs/plat/socionext-uniphier.rst
-- plat/socionext/\*
+- plat/socionext/uniphier/\*
 
 Xilinx platform sub-maintainer
 ------------------------------