ARM: mach-uniphier: Move Socionext UniPhier support into Kconfig.platforms

This removes the need for a dedicated Kconfig and empty mach directory.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>