arch/x86/smbios_defaults.c: Default to motherboard type

Nearly every board that coreboot supports is a motherboard.

Change-Id: I1419874a0ba3f2e21568fa4b07b88f2048d10203
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50180
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/smbios_defaults.c

@@ -69,7 +69,7 @@ __weak const char *smbios_mainboard_location_in_chassis(void)
 
 __weak smbios_board_type smbios_mainboard_board_type(void)
 {
-	return SMBIOS_BOARD_TYPE_UNKNOWN;
+	return SMBIOS_BOARD_TYPE_MOTHERBOARD;
 }
 
 __weak void smbios_ec_revision(uint8_t *ec_major_revision, uint8_t *ec_minor_revision)