powerpc/85xx: P2020: Add law_trgt_if property to PCIe DT nodes

DT law_trgt_if property defines Local Access Window Target Interface.

Local Access Window Target Interface is used for identifying individual
peripheral and mapping its memory to CPU. Interface id is defined by
hardware itself.

U-Boot uses law_trgt_if DT property in PCIe nodes for configuring memory
mapping of individual PCIe controllers.

Linux kernel fsl_pci.c driver currently does not touch Local Access Window
and expects that U-Boot configures it properly.

Add law_trgt_if property to PCIe DT nodes for P2020. This allows usage of
kernel P2020 PCIe DT nodes in U-Boot. And therefore allows to share P2020
DTS files between Linux kernel and U-Boot.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220504180822.29782-1-pali@kernel.org