mb/google/rex/Kconfig: Add SMBIOS mainboard version flag

Add GOOGLE_SMBIOS_MAINBOARD_VERSION flag for rex board.

BUG=None
TEST=Verfied board ID for rex using "crossystem" command, giving the
     output as 1.

Without CL:
localhost ~ # crossystem
arch                    = x86       # [RO/str] Platform architecture
backup_nvram_request    = 1         # [RW/int] Backup the nvram somewh
battery_cutoff_request  = 0         # [RW/int] Cut off battery and shu
block_devmode           = 0         # [RW/int] Block all use of develo
board_id                = (error)   # [RO/int] Board hardware revision
clear_tpm_owner_done    = 0         # [RW/int] Clear TPM owner done

With CL:
localhost ~ # crossystem
arch                    = x86       # [RO/str] Platform architecture
backup_nvram_request    = 1         # [RW/int] Backup the nvram somewh
battery_cutoff_request  = 0         # [RW/int] Cut off battery and shu
block_devmode           = 0         # [RW/int] Block all use of develo
board_id                = 1         # [RO/int] Board hardware revision
clear_tpm_owner_done    = 0         # [RW/int] Clear TPM owner done

Signed-off-by: Jay Patel <jay2.patel@intel.com>
Change-Id: I644ed7a948f0094a0be080153d83eaa2e37b8f1f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74037
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/rex/Kconfig

@@ -17,6 +17,7 @@ config BOARD_GOOGLE_REX_COMMON
 	select EC_GOOGLE_CHROMEEC_SKUID
 	select FW_CONFIG
 	select FW_CONFIG_SOURCE_CHROMEEC_CBI
+	select GOOGLE_SMBIOS_MAINBOARD_VERSION
 	select HAVE_ACPI_RESUME
 	select HAVE_ACPI_TABLES
 	select I2C_TPM