Build: Generate header file from NS agent mailbox manifest

NS agent mailbox manifest allows mapping of various client ID ranges
to the mailbox interrupts. Generate header file with this information
so that it can be used externally.

Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I1b70af0017e8153d743c61687d9bf0ad385d539f