arm64: dts: broadcom: Enable device-tree overlay support for RPi devices

arm64: dts: broadcom: Enable device-tree overlay support for RPi devices

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 v6.4-rc1 increase in size is as follows:

  bcm2711-rpi-400.dtb       27556 -> 38141 bytes
  bcm2711-rpi-4-b.dtb       27484 -> 38069 bytes
  bcm2711-rpi-cm4-io.dtb    27373 -> 38076 bytes
  bcm2837-rpi-3-a-plus.dtb  14930 -> 20713 bytes
  bcm2837-rpi-3-b.dtb       15107 -> 20979 bytes
  bcm2837-rpi-3-b-plus.dtb  15463 -> 21443 bytes
  bcm2837-rpi-cm3-io3.dtb   14429 -> 20098 bytes
  bcm2837-rpi-zero-2-w.dtb  14781 -> 20524 bytes

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Link: https://lore.kernel.org/r/20220410225940.135744-2-aurelien@aurel32.net
[ukleinek: rebased to v6.4, replaced by a single assignment to DTC_FLAGS]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>