treewide,intel/skylake: Use boolean type for s0ix_enable dt option

Using the boolean type and the true/false macros give the reader a
better understanding about the option. Thus, use the bool type for the
attribute and use the macros for assignments.

Skylake mainboards which use that option were changed by the following
command ran from the root directory.

    socs="SOC_INTEL_(SKYLAKE|KABYLAKE|SKYLAKE_LGA1151_V2)" && \
    option="s0ix_enable" && \
    grep -Er "${socs}" src/mainboard | \
        cut -d ':' -f 1 | \
        awk -F '[/]' '{print $1"/"$2"/"$3"/"$4}' | \
        xargs grep -r "${option}" | \
        cut -d ':' -f 1 | \
        xargs sed -i'' -e "s/${option}\".*\=.*\"1\"/${option}\" \= true/g"

Change-Id: I372dfb65e6bbfc79c3f036ce34bc399875d5ff16
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75871
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jan Samek <jan.samek@siemens.com>
Reviewed-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
diff --git a/src/mainboard/google/fizz/variants/baseboard/devicetree.cb b/src/mainboard/google/fizz/variants/baseboard/devicetree.cb
index 74c601a..e19e2386 100644
--- a/src/mainboard/google/fizz/variants/baseboard/devicetree.cb
+++ b/src/mainboard/google/fizz/variants/baseboard/devicetree.cb
@@ -62,7 +62,7 @@
 	register "dptf_enable" = "1"
 
 	# Enable S0ix
-	register "s0ix_enable" = "1"
+	register "s0ix_enable" = true
 
 	# FSP Configuration
 	register "SataSalpSupport" = "0"