intel/common/pmc: Add functions for IPC mailbox in ACPI

This change adds two functions that provide an IPC mailbox method via
ACPI for runtime clock configuration.

pmc_acpi_fill_ssdt_ipc_write_method() will provide a method in the SSDT
that can be called by other ACPI devices to send an IPC mailbox command.
This function is exported because some SOCs override the default PMC
device and need to call this function to write the method into the SSDT.

pmc_acpi_set_pci_clock() will call the method defined by the previous
function to enable or disable the PCIe SRCCLK for a specified root port
and clock pin.  It can be called by the PCIe root port after turning off
power to the attached device.

BUG=b:160996445
TEST=boot on volteer device and disassemble the SSDT to ensure that this
method exists.

Signed-off-by: Duncan Laurie <dlaurie@google.com>
Change-Id: I95f5a1ba2bc6905e0f8ce0e8b2342ad1287a23a0
---
M src/soc/intel/common/block/include/intelblocks/pmc_ipc.h
M src/soc/intel/common/block/pmc/Kconfig
M src/soc/intel/common/block/pmc/Makefile.inc
M src/soc/intel/common/block/pmc/pmc_ipc.c
4 files changed, 195 insertions(+), 3 deletions(-)