BeagleBoardPkg: switch to use MdeModulePkg ResetSystemLib
    
    The BeagleBoard port used EmbeddedPkg/ResetRuntimeDxe/ResetRuntimeDxe.inf
    for its reset handling. With the arrival
    MdeModulePkg/Universal/ResetSystemRuntimeDxe/ResetSystemRuntimeDxe.inf
    
    As part of this, change BeagleBoardPkg/Library/ResetSystemLib to be an
    implementation of ResetSystemLib instead of the previous
    EfiResetSystemLib.
    
    Wire all reset variants to ResetCold, except for ResetShutdown and
    EnterS3WithImmediateWake, which return immediately.
    
    Note: this ResetSystemLib never supported being called after
    ExitBootservices, and this shortcoming is not addressed here.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
    Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>