MAINTAINERS: Update ARM/UniPhier SoCs maintainers and status

Mark the UniPhier platform maintained and Masami Hiramatsu and
Kunihiko Hayashi are taking over maintainership of the UniPhier platform.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/1617986171-20346-1-git-send-email-hayashi.kunihiko@socionext.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>