infra/common: add user guide for uefi secure boot

Add a new document that explains the procedure to validate uefi secure
boot on the Neoverse reference design platforms.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Change-Id: Ifb2d5e0225a0a448273c111dcfc96a6d5d06b2cb