ArmVirtPkg: Add RngLib based on TimerLib for CryptoPkg

Updates the DSC for the ArmVirtPkg platform to add a RngLib that uses the
TimerLib. This is due to a later change that adds TimerLib as a dependency
for OpenSSL. The TimerLib based RngLib mimics the behavior of OpenSSL
previously and it is recommended to switch to a better source of
entropy than the system's performance counter.

Ref: https://github.com/tianocore/edk2/pull/845
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1871



Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Matthew Carlson <matthewfcarlson@gmail.com>