usb: ehci-fsl: set INCR8 mode for system bus interface on MPC512x

Use INCR8 mode for system bus interface of the USB controller
on MPC512x. This is a work-around for the AHB bus lock up
problem observed on MPC512x when there is heavy simultaneous
PATA write or network (FEC) activity.

See also "12.4 The USB controller can issue transactions that lock
up the AHB bus under certain conditions" in MPC5121e (M36P) Errata.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Matthias Fuchs <matthias.fuchs@esd.ue>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>