ppc: Move brg_clk to arch_global_data

Move this field into arch_global_data and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/lib/board.c b/arch/powerpc/lib/board.c
index 6a7bf4b..b1069a6 100644
--- a/arch/powerpc/lib/board.c
+++ b/arch/powerpc/lib/board.c
@@ -581,7 +581,7 @@
 	bd->bi_busfreq = gd->bus_clk;	/* Bus Freq,      in Hz */
 #if defined(CONFIG_CPM2)
 	bd->bi_cpmfreq = gd->cpm_clk;
-	bd->bi_brgfreq = gd->brg_clk;
+	bd->bi_brgfreq = gd->arch.brg_clk;
 	bd->bi_sccfreq = gd->scc_clk;
 	bd->bi_vco = gd->vco_out;
 #endif /* CONFIG_CPM2 */