linux-firmware: Add i915 DMC firmware

DMC provides additional graphics low-power idle states. It provides
capability to save and restore display registers across these low-power
states independently from the OS/Kernel.

This is the first release of DMC firmware for Skylake platforms.

skl_dmc_ver1.bin is a symbolik link to latest recommended minor release.
This major version will be just upgraded on code when it is required
software changes for API/ABI compatibility.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>