drm/i915/guc: support bigger RSA keys

Some of the newer HW will use bigger RSA keys to authenticate the GuC
binary. On those platforms the HW will read the key from memory instead
of the RSA registers, so we need to copy it in a dedicated vma, like we
do for the HuC. The address of the key is provided to the HW via the
first RSA register.

v2: clarify that the RSA behavior is hardcoded in the bootrom (Matt)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211211000756.1698923-4-daniele.ceraolospurio@intel.com