doc: arm: cortex_m: Update feature table

Add Cortex-M85 under the Arm v8.1-M architecture variant column.
Cortex-M85 shares feature support with Cortex-M55, including Privileged
Execute Never (PXN), and additionally introduces Pointer Authentication
and Branch Target Identification (PACBTI) extensions.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>