sunxi: Revert "sunxi: make SoC variant choice mandatory"

This reverts commit 1a5f0de0("sunxi: make SoC variant choice
mandatory").

With the optional marking in the Kconfig "make savedefconfig"
will drop CONFIG_MACH_SUN4I=y from all the A10 boards, making it
hard to see at a glance which family of sunxi chips the defconfig
is for.

This commit therefore restores the optional, and restores
CONFIG_MACH_SUN4I=y to all defconfig's which had it dropped
because of this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>