sunxi: power: Compile sy8106a driver only during SPL build

Driver for that regulator is used only in SPL and it uses old I2C
interface. If we want to use DM I2C in U-Boot proper, compilation of
this driver has to be limited only to SPL.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>