dm: pci: Add APIs to find capability and extended capability

This introduces two new APIs dm_pci_find_capability() and
dm_pci_find_ext_capability() to get PCI capability address and
PCI express extended capability address for a given PCI device.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>