tree: Use boolean for s0ix_enable

Change-Id: Id0ab5e641684e03da555a127808c0def5a53cbe6
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84159
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/system76/tgl-u/devicetree.cb

@@ -15,7 +15,7 @@ chip soc/intel/tigerlake
 	register "eist_enable" = "true"
 
 	# Enable s0ix, required for TGL-U
-	register "s0ix_enable" = "1"
+	register "s0ix_enable" = "true"
 
 # FSP Memory (soc/intel/tigerlake/romstage/fsp_params.c)
 	# Enable C6 DRAM