sunxi: Convert CONS_INDEX to Kconfig

Convert the CONS_INDEX configuration to Kconfig.
Update sunxi's defconfigs to remove SYS_EXTRA_OPTIONS variable not
needed anymore.
Default value is 1 except for sun5i (equals 2) and sun8i (equals 5).

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
[Maxime: Added a depends on ARCH_SUNXI to avoid build breakages]
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>