ARC: add power management options

Even though ARC cores itself don't have any power management except SLEEP
state it's possible to use power management features of selected peripherals.

For example USB OTG requires PM_RUNTIME which is only available if
kernel/power/Kconfig is sourced by architecture.
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/Kconfig

@@ -428,3 +428,4 @@ source "arch/arc/Kconfig.debug"
 source "security/Kconfig"
 source "crypto/Kconfig"
 source "lib/Kconfig"
+source "kernel/power/Kconfig"