mb/google/volteer: Switch to using device pointers using alias names

This change replaces the device tree walks with device pointers by
using alias names for the following devices:

1. PMC MUX connector
2. SPI TPM
3. I2C TPM

Change-Id: I38f87d3a90a7253f2a29aba7db9a9f9744985494
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57740
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
diff --git a/src/mainboard/google/volteer/variants/baseboard/devicetree.cb b/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
index 0d067e6..b4c4f6a 100644
--- a/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
+++ b/src/mainboard/google/volteer/variants/baseboard/devicetree.cb
@@ -508,7 +508,7 @@
 				register "hid" = "ACPI_DT_NAMESPACE_HID"
 				register "compat_string" = ""google,cr50""
 				register "irq" = "ACPI_IRQ_EDGE_LOW(GPP_C21_IRQ)"
-				device spi 0 on end
+				device spi 0 alias spi_tpm on end
 			end
 		end
 		device ref gspi1 on