ARM: dts: at91: Enable device-tree overlay support for AT91 boards

Add the '-@' DTC option for AT91 boards that have device-tree overlays.
This option populates the '__symbols__' node that contains all the
necessary symbols for supporting device-tree overlays (from bootloader)
on these devices.

This change increases the size of the resulting DTB with ~30%-40%. Below
are the measurements performed v6.4-rc6:

at91-sam9x60_curiosity.dtb	28499 -> 36641 bytes
at91-sam9x60ek.dtb		30867 -> 39609 bytes
at91-sama5d27_som1_ek.dtb	26086 -> 34166 bytes
at91-sama5d27_wlsom1_ek.dtb	26045 -> 33184 bytes
at91-sama5d2_icp.dtb		27521 -> 36155 bytes
at91-sama5d2_ptc_ek.dtb		23237 -> 29612 bytes
at91-sama5d2_xplained.dtb	27262 -> 35326 bytes
at91-sama5d3_eds.dtb		27924 -> 39939 bytes
at91-sama5d3_xplained.dtb	28400 -> 40650 bytes
at91-sama5d4_xplained.dtb	26456 -> 36107 bytes
at91-sama7g5ek.dtb		29212 -> 37289 bytes

Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
[claudiu.beznea: s/arm: dts/ARM: dts: at91/ in commit title]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230616152932.1484154-1-cristian.birsan@microchip.com

Author: cristibirsan

arch/arm/boot/dts/Makefile

@@ -50,9 +50,21 @@ dtb-$(CONFIG_SOC_AT91SAM9) += \
 	at91sam9g35ek.dtb \
 	at91sam9x25ek.dtb \
 	at91sam9x35ek.dtb
+# Enables support for device-tree overlays
+DTC_FLAGS_at91-sam9x60_curiosity := -@
+DTC_FLAGS_at91-sam9x60ek := -@
 dtb-$(CONFIG_SOC_SAM9X60) += \
 	at91-sam9x60_curiosity.dtb \
 	at91-sam9x60ek.dtb
+# Enables support for device-tree overlays
+DTC_FLAGS_at91-sama5d27_som1_ek := -@
+DTC_FLAGS_at91-sama5d27_wlsom1_ek := -@
+DTC_FLAGS_at91-sama5d2_icp := -@
+DTC_FLAGS_at91-sama5d2_ptc_ek := -@
+DTC_FLAGS_at91-sama5d2_xplained := -@
+DTC_FLAGS_at91-sama5d3_eds := -@
+DTC_FLAGS_at91-sama5d3_xplained := -@
+DTC_FLAGS_at91-sama5d4_xplained := -@
 dtb-$(CONFIG_SOC_SAM_V7) += \
 	at91-kizbox2-2.dtb \
 	at91-kizbox3-hs.dtb \
@@ -79,6 +91,8 @@ dtb-$(CONFIG_SOC_SAM_V7) += \
 	at91-sama5d4_xplained.dtb \
 	at91-sama5d4ek.dtb \
 	at91-vinco.dtb
+# Enables support for device-tree overlays
+DTC_FLAGS_at91-sama7g5ek := -@
 dtb-$(CONFIG_SOC_SAMA7G5) += \
 	at91-sama7g5ek.dtb
 dtb-$(CONFIG_SOC_SP7021) += \