SDEI: Add prioritisation clarification

To make exception handling amongst Secure components, require that SDEI
exception priorities must be assigned the lowest among Secure
priorities. Clarify documentation to this effect.

Change-Id: I92524b7b7e9b3fa06a10c86372bc3c4dd18c00ad
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>