With SCU standby enabled, SCU CLK will be turned off when all processors
are in WFI mode.  And the clock will be turned on when any processor
leaves WFI mode.

This behavior should be preferable in terms of power efficiency of
system idle.  So let's set the SCU standby bit to enable the support in
function scu_enable().

Cortex-A9 earlier than r2p0 has no standby bit in SCU, so we need to
skip setting the bit for those.
Signed-off-by: NShawn Guo <shawn.guo@freescale.com>
Acked-by: NSoren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>