ARM: OMAP2+: Add HAVE_ARM_SCU for AM43XX

ARM: OMAP2+: Add HAVE_ARM_SCU for AM43XX

CONFIG_HAVE_ARM_SCU only gets selected if CONFIG_SMP is selected in an OMAP
system, however AM43XX needs this option regardless of CONFIG_SMP and also
for an AM43XX only build as it is important for controlling power in the SoC.
Without this we cannot suspend the CPU for SoC suspend or cpuidle. The
ARM Cortex A9 needs SCU CPU Power Status bits to be set to off mode in order
for the PRCM to transition the MPU to low power modes.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>