mb/intel/mtlrvp: Create baseboard structure for mtlrvp

This patch will create the baseboard structure for mtlrvp. Changes
include,
1. Adding Baseboard config for mtlrvp in Kconfig
2. Move gpio.h to corresponding baseboard directory
3. Append header reference to CPPFLAGS_common in Makefile.inc

BUG=none
TEST=FW_NAME=mtlrvp_p emerge-rex coreboot chromeos-bootimage

Signed-off-by: Harsha B R <harsha.b.r@intel.com>
Change-Id: I82acb6879fecb242014258f2c358804d5abbbd48
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69971
Reviewed-by: Rizwan Qureshi <rizwan.qureshi@intel.com>
Reviewed-by: Ronak Kanabar <ronak.kanabar@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/intel/mtlrvp/Kconfig

@@ -13,6 +13,10 @@ if BOARD_INTEL_MTLRVP_COMMON
 config MAINBOARD_DIR
 	default "intel/mtlrvp"
 
+config BASEBOARD_DIR
+	string
+	default "mtlrvp"
+
 config MAINBOARD_PART_NUMBER
 	string
 	default "mtlrvp"

src/mainboard/intel/mtlrvp/Makefile.inc

@@ -1,5 +1,9 @@
 ## SPDX-License-Identifier: GPL-2.0-or-later
 
-CPPFLAGS_common += -I$(src)/mainboard/$(MAINBOARDDIR)/include
+VARIANT_DIR:=$(call strip_quotes,$(CONFIG_VARIANT_DIR))
+BASEBOARD_DIR:=$(call strip_quotes,$(CONFIG_BASEBOARD_DIR))
 
+subdirs-y += variants/baseboard/$(BASEBOARD_DIR)
 subdirs-y += variants/$(VARIANT_DIR)
+
+CPPFLAGS_common += -I$(src)/mainboard/$(MAINBOARDDIR)/variants/baseboard/$(BASEBOARD_DIR)/include