maintainers: step down as code owner of UniPhier platform
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 <378b411a8a63ecc7605ec4272234862d7781c6ae@socionext.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware