ArmPlatformPkg/Bds: Reduce boot device entries

If a storage device is used for Variable storage or access is provided
by the Simple Filesystem Protocol do not list it again for RAW memory
map access in the 'Add Boot device' menu.

Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>

Change-Id: I47b8e32d1f4b21d7c8701f646870643b5fb86716