powerpc/boot/dts: Fix dtc "pciex" warnings

With CONFIG_OF_ALL_DTBS=y, as set by eg. allmodconfig, we see lots of
warnings about our dts files, such as:

  arch/powerpc/boot/dts/glacier.dts:492.26-532.5:
  Warning (pci_bridge): /plb/pciex@d00000000: node name is not "pci"
  or "pcie"

The node name should not particularly matter, it's just a name, and
AFAICS there's no kernel code that cares whether nodes are *named*
"pciex" or "pcie". So shutup these warnings by converting to the name
dtc wants.

As always there's some risk this could break something obscure that
does rely on the name, in which case we can revert.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200623130320.405852-1-mpe@ellerman.id.au