PCI: brcmstb: Change RCB_{MPS,64B}_MODE bits

Upstream commit [1] unconditionally sets the RCB_MPS and RCB_64B bits
that govern where packets are split. We think this is potentially
harmful, particularly on CM4 and Pi 5 where potentially any PCIe devices
could be attached.

Make RCB_MPS conditional on a DT property and never set RCB_64B.

[1] commit 602fb860 ("PCI: brcmstb: Set RCB_{MPS,64B}_MODE bits")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>