maintainers: step down as code owner of UniPhier platform

I am leaving Socionext. Orphan the UniPhier platform until somebody
takes the role.

Change-Id: I54d3da6d49c1ccaaa475431654db578b683db88a
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

docs/about/maintainers.rst

@@ -523,8 +523,7 @@ Texas Instruments platform port
 
 UniPhier platform port
 ^^^^^^^^^^^^^^^^^^^^^^
-:M: Masahiro Yamada <yamada.masahiro@socionext.com>
-:G: `masahir0y`_
+:M: Orphan
 :F: docs/plat/socionext-uniphier.rst
 :F: plat/socionext/uniphier/