sunxi: Only enable i2c support in the SPL when needed

We do not need i2c support in the SPL when there is no PMIC (some sun4i
boards), or when the PMIC is not using i2c such as on sun6i and sun8i.

This reduces the SPL size from (e.g.) 21812 to 19260 bytes.

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