ppc: Remove extra pci_clk fields from global_data

PPC has several of these fields, selected by chip type, although only one
is ever compiled in.

Instead, use a single field. It would be nice if this could be selected
by CONFIG_PCI, but some chips (e.g. mpc5xxx) use pci_clk even when
CONFIG_PCI is not enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>