drivers: bcm283x: Set pre-location flag for OF_BOARD

drivers: bcm283x: Set pre-location flag for OF_BOARD

U-Boot support on Raspberry Pi 4 relies on the device-tree
provided by the firmware. The blob does not contain the
U-Boot specific pre-loc-rel properties. The result is, that
the U-Boot banner is not printed.

We fix this by setting the DM_FLAG_PRE_RELOC flag in the driver,
if we rely on a device-tree provided by the firmware.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>