riscv: dts: Enable device-tree overlay support for starfive devices

Add the '-@' DTC option for the starfive 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 starfive devices allow various modules to 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 ~20%. For example, with v6.4 increase in size is as follows:

jh7100-beaglev-starlight.dtb 6192 -> 7339
jh7100-starfive-visionfive-v1.dtb 6281 -> 7428
jh7110-starfive-visionfive-2-v1.2a.dtb 11101 -> 13447
jh7110-starfive-visionfive-2-v1.3b.dtb 11101 -> 13447
Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Acked-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/starfive/Makefile

 # SPDX-License-Identifier: GPL-2.0
+# Enables support for device-tree overlays
+DTC_FLAGS_jh7100-beaglev-starlight := -@
+DTC_FLAGS_jh7100-starfive-visionfive-v1 := -@
+DTC_FLAGS_jh7110-starfive-visionfive-2-v1.2a := -@
+DTC_FLAGS_jh7110-starfive-visionfive-2-v1.3b := -@
+
 dtb-$(CONFIG_ARCH_STARFIVE) += jh7100-beaglev-starlight.dtb
 dtb-$(CONFIG_ARCH_STARFIVE) += jh7100-starfive-visionfive-v1.dtb