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