i2c: imc-skylake: add driver for Intel Skylake-X iMC SMBus engine

Add a driver for the integrated memory controller (iMC) SMBus engine on
Intel Skylake-X / Cascade Lake-X processors (socket LGA 2066, platform
X299, PCU function 8086:2085).

The engine provides two SMBus channels — one per pair of DIMM slots —
over which SPD EEPROMs, DDR4 thermal sensors and third-party LED
controllers are accessible. Exposing it as a pair of standard Linux I2C
adapters lets existing tools (i2c-tools, lm-sensors) use it without
bespoke sysfs hacks.

Key design decisions:
- ECAM MMIO access instead of CF8/CFC port I/O (SMM traps port writes)
- Dynamic MCFG table parsing for mmcfg_base (no hardcoding)
- Support for SMBus BYTE_DATA and WORD_DATA transfers
- devm-managed resources with automatic cleanup
- Global mutex to serialize transactions across both channels sharing
  the same ECAM mapping

Signed-off-by: Simone Chifari <simone.chifari@gmail.com>

docs/submission/Kconfig.kernel

@@ -0,0 +1,18 @@
+config I2C_IMC_SKYLAKE
+	tristate "Intel Skylake-X iMC SMBus adapter"
+	depends on PCI && ACPI && X86
+	help
+	  Say Y here if you want kernel support for the integrated memory
+	  controller (iMC) SMBus engine found in Intel Skylake-X / Cascade
+	  Lake-X processors (socket LGA 2066, platform X299, PCU function
+	  8086:2085).
+
+	  The engine is exposed through the PCI configuration space of the
+	  PCU function and is driven via ECAM MMIO (not CF8/CFC port I/O,
+	  which is trapped by System Management Mode on this platform).
+	  Two I2C adapters are registered, one per hardware SMBus channel,
+	  allowing access to DDR4 DIMM SPD EEPROMs (0x50-0x57) and thermal
+	  sensors from userspace via standard i2c-tools and lm-sensors.
+
+	  If unsure, say N.  This driver is only useful on Intel X299 desktop
+	  / HEDT systems with the Skylake-X or Cascade Lake-X CPU.