infra/common: document WinPE boot

Add a new document that explains the procedure to boot WinPE on the
Neoverse Reference Design platforms.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Ic0a0a03a660fece2f38a1cb38f6b6da932c353e6