powerpc: dts: turris1x.dts: Fix PCIe MEM size for pci2 node

Freescale PCIe controllers on their PCIe Root Ports do not have any
mappable PCI BAR allocate from PCIe MEM.

Information about 1MB window on BAR0 of PCIe Root Port was misleading
because Freescale PCIe controllers have at BAR0 position different register
PEXCSRBAR, and kernel correctly skipts BAR0 for these Freescale PCIe Root
Ports.

So update comment about P2020 PCIe Root Port and decrease PCIe MEM size
required for PCIe controller (pci2 node) on which is on-board xHCI
controller.

lspci confirms that on P2020 PCIe Root Port is no PCI BAR and /proc/iomem
sees that only c0000000-c000ffff and c0010000-c0011fff ranges are used.

Fixes: 54c15ec3

 ("powerpc: dts: Add DTS file for CZ.NIC Turris 1.x routers")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230505172818.18416-1-pali@kernel.org