Currently the fixup code assumes that only one i2c bus is active. If we
reuse the board code for other boards this isn't always true. Fix that
by using of_find_i2c_adapter_by_node() to address both cases.

Fixes: b6bebb7aca ("ARM: i.MX: phytec-som-imx6: phycore: Set BUCK mode to SYNC")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>