pci: Make auto-config code a little more robust

On my DS414, some PCI devices return odd values when probing BAR sizes.
An obvious case is all-ones response, the Linux driver
(drivers/pci/probe.c) catches those explicitly and a comment explains
that either bit 0 or bit 1 must be clear (depending on MEM or IO type).
Other BARs return e.g. 0xfff0000f or 0xfff00004 and thus manage to break
size calculation due to the "middle" zeroes. Mitigate that copying more
or less what Linux does and do a "find least bit set".

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>