sunxi: Move function for later convenience

This commit only moves i2c_init_board() function almost to the top and
doesn't have any functional changes.

This is needed for a temporary workaround in next commit when support
for DM I2C will be introduced.

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>