{soc/intel/cmn/pcie, mb/google/volteer}: Rename `is_external` variable

Name a variable based on its utility. `is_external` variable adds
`ExternalFacingPort` _DSD property to an ACPI device hence
rename it to `add_acpi_external_facing_port`.

BUG=b:259716145
TEST=Build google/rex with this flag and verify it in SSDT at
runtime.

SSDT snippet:
   Name (_DSD, Package (0x04)  // _DSD: Device-Specific Data
   {
       ToUUID ("6211e2c0-58a3-4af3-90e1-927a4e0c55a4"),
       Package (0x01)
       {
           Package (0x02)
           {
               "HotPlugSupportInD3",
               One
           }
       },

       ToUUID ("efcc06cc-73ac-4bc3-bff0-76143807c389"),
       Package (0x01)
       {
           Package (0x02)
           {
               "ExternalFacingPort",
               One
           }
        }
    })

Signed-off-by: Kapil Porwal <kapilporwal@google.com>
Change-Id: I65100283ed9b65037c9890f28ecab41fcfa25d83
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69970
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/google/volteer/variants/baseboard/devicetree.cb b/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
index 9f3987e..3a54d3b 100644
--- a/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
+++ b/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
@@ -489,7 +489,7 @@
 			chip soc/intel/common/block/pcie/rtd3
 				register "reset_gpio" = "ACPI_GPIO_OUTPUT_ACTIVE_LOW(GPP_H3)"
 				register "srcclk_pin" = "3"
-				register "is_external" = "1"
+				register "add_acpi_external_facing_port" = "1"
 				device generic 1 on
 					probe DB_SD SD_RTS5261
 				end