allwinner: a64: power: Make sunxi_turn_off_soc static

The function is only used in this file, and it doesn't make sense for it
to be used anywhere else.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: Iab18f082911edcdbc37ceeaff8c512be68e0cb0f
diff --git a/plat/allwinner/sun50i_a64/sunxi_power.c b/plat/allwinner/sun50i_a64/sunxi_power.c
index b6437ea..d14ced9 100644
--- a/plat/allwinner/sun50i_a64/sunxi_power.c
+++ b/plat/allwinner/sun50i_a64/sunxi_power.c
@@ -39,7 +39,7 @@
  * disabled.
  * This function only cares about peripherals.
  */
-void sunxi_turn_off_soc(uint16_t socid)
+static void sunxi_turn_off_soc(uint16_t socid)
 {
 	int i;