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>

dkms.conf

@@ -0,0 +1,10 @@
+PACKAGE_NAME="i2c-imc-skylake"
+PACKAGE_VERSION="1.0"
+
+BUILT_MODULE_NAME[0]="i2c-imc-skylake"
+DEST_MODULE_LOCATION[0]="/updates"
+
+MAKE[0]="make KVER=${kernelver}"
+CLEAN="make clean KVER=${kernelver}"
+
+AUTOINSTALL="yes"