global_data: Move pci_clk to m68k and powerpc

Only m68k and powerpc use this field, so move it to the arch-specific
info, to reduce the size for other archs.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/cpu/mpc83xx/speed.c b/arch/powerpc/cpu/mpc83xx/speed.c
index 7246496..0185ab5 100644
--- a/arch/powerpc/cpu/mpc83xx/speed.c
+++ b/arch/powerpc/cpu/mpc83xx/speed.c
@@ -456,7 +456,7 @@
 #if defined(CONFIG_ARCH_MPC837X)
 	gd->arch.sata_clk = sata_clk;
 #endif
-	gd->pci_clk = pci_sync_in;
+	gd->arch.pci_clk = pci_sync_in;
 	gd->cpu_clk = gd->arch.core_clk;
 	gd->bus_clk = gd->arch.csb_clk;
 	return 0;