fix(boards): Add xiao_ble sd_partition label for nosd snippet compat

Upstream xiao_ble uses different naming convention for the partition
labels, so add an additional label for the SD range, so the existing
nrf52840-nosd snippet will still work with the board.

app/boards/extensions/xiao_ble/xiao_ble.overlay

@@ -6,6 +6,9 @@
 
 #include <common/nordic/nrf52840_uf2_boot_mode.dtsi>
 
+/* Add an sd_partition label for compatibility with the nosd snippet */
+sd_partition: &reserved_partition_0 { };
+
 / {
     chosen {
         zmk,battery = &vbatt;