doc: Make checkpatch ignore rst files

Previously checkpatch was invoked with options to make it ignore
Markdown (md) files as this was the dominant format for TF-A
documents. Now that rst is being used everywhere this option needs
updating.

Change-Id: I59b5a0bcc45d2386df4f880b8d333baef0bbee77
Signed-off-by: Paul Beesley <paul.beesley@arm.com>