drm/i915/pxp: define PXP device flag and kconfig

Ahead of the PXP implementation, define the relevant define flag and
kconfig option.

v2: flip kconfig default to N. Some machines have IFWIs that do not
support PXP, so we need it to be an opt-in until we add support to query
the caps from the mei device.

v10: change comments from "Gen12+" to "Gen12 and newer"

Signed-off-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924191452.1539378-4-alan.previn.teres.alexis@intel.com