allwinner: Build PMIC bus drivers only in BL31

These are used by the PMIC setup code, which runs during BL31
initialization, and the PSCI shutdown code, also a part of BL31.
They can't be needed before BL31, or it wouldn't be possible to boot.
Allwinner platforms don't generally build anything but BL31 anyway, but
this change improves clarity and consistency with allwinner-common.mk.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: I24f1d9ca8b4256e44badf5218d04d8690082babf

plat/allwinner/sun50i_a64/platform.mk

 #
-# Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -7,4 +7,4 @@
 # The differences between the platform are covered by the include files.
 include plat/allwinner/common/allwinner-common.mk
 
-PLAT_BL_COMMON_SOURCES	+=	drivers/allwinner/sunxi_rsb.c
+BL31_SOURCES		+=	drivers/allwinner/sunxi_rsb.c

plat/allwinner/sun50i_h6/platform.mk

 #
-# Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -7,4 +7,4 @@
 # The differences between the platform are covered by the include files.
 include plat/allwinner/common/allwinner-common.mk
 
-PLAT_BL_COMMON_SOURCES	+=	drivers/mentor/i2c/mi2cv.c
+BL31_SOURCES		+=	drivers/mentor/i2c/mi2cv.c