CI: labeler: add sifiveu target

Add support for 'sifiveu' target and its specific packages in labeler.

Signed-off-by: Piotr Dymacz <[email protected]>

.github/labeler.yml

@@ -94,6 +94,10 @@
   - "target/linux/rockchip/**"
   - "package/boot/arm-trusted-firmware-rockchip/**"
   - "package/boot/uboot-rockchip/**"
+"target/sifiveu":
+  - "target/linux/sifiveu/**"
+  - "package/boot/uboot-sifiveu/**"
+  - "package/boot/opensbi/**"
 "target/sunxi":
   - "target/linux/sunxi/**"
   - "package/boot/arm-trusted-firmware-sunxi/**"