dm: fix probing of all devices that have u-boot, dm-pre-reloc in SPL/TPL

Currently, dm_probe_devices checks that the flags of the device contains
DM_FLAG_PRE_RELOC. However DM_FLAG_PRE_RELOC is a driver - and not a
device - flag. This means that the check in pre_reloc_only mode would
always fail.

Instead, what was aimed to be checked is that either the driver of the
device has the flag set, or that the device has the u-boot,dm-pre-reloc
Device Tree property set.

So let's fix the check to allow u-boot,dm-pre-reloc devices to be
probed.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>