ARM: imx: remove imx_scu_standby_enable()

With commit c716483c ("ARM: 8122/1: smp_scu: enable SCU standby
support"), the STANDBY bit of SCU is handled by core function
scu_enable().  So imx_scu_standby_enable() can be removed now.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>