[efi] Mark SNP devices as children of EFI PCI device

[efi] Mark SNP devices as children of EFI PCI device

Re-open the EFI_PCI_IO_PROTOCOL specifying an Attributes value of
EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.  This causes the SNP devices to
be marked as children of the EFI PCI device (as shown in the "devtree"
command).

On at least one IBM blade system, this is required in order to have
the relevant drivers automatically attach to the SNP controller at
device creation time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>