pci: coreboot: Don't read regions when booting

When U-Boot is the second-stage bootloader, PCI is already set up. We
cannot read the regions from the device tree. There is no point anyway,
since PCI devices have already been allocated according to the regions
and it is not safe for U-Boot to make any changes.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Fixes: f2ebaaa9 ("pci: Handle failed calloc in decode_regions()")
Tested-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>