sunxi: remove usage of machine_is_sunNi() in favour of sunxi_is_sunNi()

Signed-off-by: Alejandro Mery <amery@geeks.cl>
sunxi:soc-detect: introduce CONFIG_SUNXI_MULTIPLATFORM

to deal with CONFIG_ARCH_SUNnI optimizations, when disabled.

Signed-off-by: Alejandro Mery <amery@geeks.cl>