arch: x86: add IPC mailbox accessor function and add SoC register access

- Exports intel_pmc_ipc() for host access to the PMC IPC mailbox
- Enables the host to access specific SoC registers through the PMC
firmware using IPC commands. This access method is necessary for
registers that are not available through direct Memory-Mapped I/O (MMIO),
which is used for other accessible parts of the PMC.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Chao Qin <chao.qin@intel.com>
Signed-off-by: Choong Yong Liang <yong.liang.choong@linux.intel.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/20250227121522.1802832-4-yong.liang.choong@linux.intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>