boards: st: stm32h745i_disco: fix CAN phy devicetree representation

On this board, the CAN transceivers are not GPIO controlled, but passive,
and should not be represented by the "can-transceiver-gpio" compatible (as
this leads to driver compilation warnings due to missing GPIO properties).

Fixes: c8faa1e3

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>