mb/purism/librem_skl: add support for 13v4/15v4 boards

Add support for Kabylake Librem 13v4/15v4 boards, reusing existing 13v2/15v3
variants since board design unchanged (only SoC).
Adjust identification strings, add Kabylake VGA PCI ID.

Change-Id: Ia795b9c7373ea2e2acd3bef309320b58e9e8449d
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Reviewed-on: https://review.coreboot.org/c/31009
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/mainboard/purism/librem_skl/Kconfig b/src/mainboard/purism/librem_skl/Kconfig
index 0c138a6..bf62bff 100644
--- a/src/mainboard/purism/librem_skl/Kconfig
+++ b/src/mainboard/purism/librem_skl/Kconfig
@@ -23,6 +23,8 @@
 	string
 	default "librem13v2" if BOARD_PURISM_LIBREM13_V2
 	default "librem15v3" if BOARD_PURISM_LIBREM15_V3
+	default "librem13v2" if BOARD_PURISM_LIBREM13_V4
+	default "librem15v3" if BOARD_PURISM_LIBREM15_V4
 
 config MAINBOARD_VENDOR
 	string
@@ -32,16 +34,22 @@
 	string
 	default "Librem 13" if BOARD_PURISM_LIBREM13_V2
 	default "Librem 15" if BOARD_PURISM_LIBREM15_V3
+	default "Librem 13" if BOARD_PURISM_LIBREM13_V4
+	default "Librem 15" if BOARD_PURISM_LIBREM15_V4
 
 config MAINBOARD_PART_NUMBER
 	string
 	default "Librem 13 v2" if BOARD_PURISM_LIBREM13_V2
 	default "Librem 15 v3" if BOARD_PURISM_LIBREM15_V3
+	default "Librem 13 v4" if BOARD_PURISM_LIBREM13_V4
+	default "Librem 15 v4" if BOARD_PURISM_LIBREM15_V4
 
 config MAINBOARD_VERSION
 	string
 	default "2.0" if BOARD_PURISM_LIBREM13_V2
 	default "3.0" if BOARD_PURISM_LIBREM15_V3
+	default "4.0" if BOARD_PURISM_LIBREM13_V4
+	default "4.0" if BOARD_PURISM_LIBREM15_V4
 
 config MAINBOARD_DIR
 	string
@@ -57,7 +65,10 @@
 
 config VGA_BIOS_ID
 	string
-	default "8086,1916"
+	default "8086,1916" if BOARD_PURISM_LIBREM13_V2
+	default "8086,1916" if BOARD_PURISM_LIBREM15_V3
+	default "8086,5916" if BOARD_PURISM_LIBREM13_V4
+	default "8086,5916" if BOARD_PURISM_LIBREM15_V4
 
 config DIMM_MAX
 	int