[PATCH RESEND 1/2] arm64: dts: broadcom: Enable device-tree overlay support for RPi devices

[Aurelien Jarno]

Add the '-@' DTC option for the Raspberry Pi devices. This option
populates the '__symbols__' node that contains all the necessary symbols
for supporting device-tree overlays (for instance from the firmware or
the bootloader) on these devices.

The Rasbperry Pi devices are well known for their GPIO header, that
allow various "HATs" or other modules do be connected and this enables
users to create out-of-tree device-tree overlays for these modules.

Please note that this change does increase the size of the resulting DTB
by ~40%. For example, with v5.17 increase in size is as follows:

  bcm2711-rpi-400.dtb       26481 -> 36830 bytes
  bcm2711-rpi-4-b.dtb       26537 -> 36886 bytes
  bcm2711-rpi-cm4-io.dtb    26426 -> 36945 bytes
  bcm2837-rpi-3-a-plus.dtb  14133 -> 19740 bytes
  bcm2837-rpi-3-b.dtb       14310 -> 20006 bytes
  bcm2837-rpi-3-b-plus.dtb  14670 -> 20474 bytes
  bcm2837-rpi-cm3-io3.dtb   13680 -> 19266 bytes

Signed-off-by: Aurelien Jarno <aurelien at aurel32.net>
---
 arch/arm64/boot/dts/broadcom/Makefile | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/arm64/boot/dts/broadcom/Makefile b/arch/arm64/boot/dts/broadcom/Makefile
index 5082fcd1fea5..0aa8ee2afdd5 100644
--- a/arch/arm64/boot/dts/broadcom/Makefile
+++ b/arch/arm64/boot/dts/broadcom/Makefile
@@ -1,4 +1,15 @@
 # SPDX-License-Identifier: GPL-2.0
+
+# Enables support for device-tree overlays
+DTC_FLAGS_bcm2711-rpi-400 := -@
+DTC_FLAGS_bcm2711-rpi-4-b := -@
+DTC_FLAGS_bcm2711-rpi-cm4-io := -@
+DTC_FLAGS_bcm2837-rpi-3-a-plus := -@
+DTC_FLAGS_bcm2837-rpi-3-b := -@
+DTC_FLAGS_bcm2837-rpi-3-b-plus := -@
+DTC_FLAGS_bcm2837-rpi-cm3-io3 := -@
+DTC_FLAGS_bcm2837-rpi-zero-2-w := -@
+
 dtb-$(CONFIG_ARCH_BCM2835) += bcm2711-rpi-400.dtb \
 			      bcm2711-rpi-4-b.dtb \
 			      bcm2711-rpi-cm4-io.dtb \
-- 
2.35.1