MAINTAINERS: step down as maintainer of UniPhier SoCs and Denali driver

I am leaving Socionext. Orphan the UniPhier platform and Denali NAND
driver until somebody takes the role.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>