ArmPlatformPkg/Bds: Check if the assumed requirements are set before to use it

ArmPlatformPkg/Bds: Check if the assumed requirements are set before to use it

There were some assumptions that some PCDs and protocols were defined.
Check if these requirements are defined; either print a debug message
to inform the user or skip the statements that require these PCDs or protocol.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11915 6f19259b-4bc3-4df7-8a09-765794883524