ARM: sunxi: move board/sunxi/Kconfig to arch/arm/mach-sunxi/Kconfig

For the consistent location of SoC-level Kconfig.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>