ppc: fix boot with sam460ex

ppc: fix boot with sam460ex

Recent changes to pcie_host corrected size of its internal region to
match what it expects: only the low 28 bits are ever decoded. Previous
code just ignored bit 29 (if size was 1 << 29) in the address which does
not make much sense.  We are now asserting on size > 1 << 28 instead,
but PPC 4xx actually allows guest to configure different sizes, and some
firmwares seem to set it to 1 << 29.

This caused e.g. qemu-system-ppc -M sam460ex to exit with an assert when
the guest writes a value to CFGMSK register when trying to map config
space. This is done in the board firmware in ppc4xx_init_pcie_port() in
roms/u-boot-sam460ex/arch/powerpc/cpu/ppc4xx/4xx_pcie.c

It's not clear what the proper fix should be but for now let's force the
size to 256MB, so anything outside the expected address range is
ignored.

Fixes: commit 1f1a7b2269 ("include/hw/pci/pcie_host: Correct PCIE_MMCFG_SIZE_MAX")
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220526224229.95183-1-mst@redhat.com>
[danielhb: changed commit msg as BALATON Zoltan suggested]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>