arm: Move uart_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/cpu/ppc4xx/4xx_uart.c b/arch/powerpc/cpu/ppc4xx/4xx_uart.c
index 38ba60b..2ab185f 100644
--- a/arch/powerpc/cpu/ppc4xx/4xx_uart.c
+++ b/arch/powerpc/cpu/ppc4xx/4xx_uart.c
@@ -296,10 +296,10 @@
 	 * the UART divisor is available
 	 */
 #ifdef CONFIG_SYS_EXT_SERIAL_CLOCK
-	gd->uart_clk = CONFIG_SYS_EXT_SERIAL_CLOCK;
+	gd->arch.uart_clk = CONFIG_SYS_EXT_SERIAL_CLOCK;
 #else
 	get_sys_info(&sys_info);
-	gd->uart_clk = sys_info.freqUART / udiv;
+	gd->arch.uart_clk = sys_info.freqUART / udiv;
 #endif
 
 	return clk;