PSCI: fix some PSCI issues and decouple CPU standby from CPU power down #325

Job	Run time
build-arm (fvp-a, GICV2)	1m 33s
build-arm (fvp-a, GICV2, CROSS_COMPILE=clang)	1m 36s
build-arm (fvp-a, GICV3)	1m 34s
build-arm (fvp-a, GICV3, CROSS_COMPILE=clang)	1m 40s
build-arm (fvp-a-aarch32, GICV2)	1m 34s
build-arm (fvp-a-aarch32, GICV2, CROSS_COMPILE=clang)	1m 33s
build-arm (fvp-a-aarch32, GICV3)	1m 34s
build-arm (fvp-a-aarch32, GICV3, CROSS_COMPILE=clang)	1m 31s
build-arm (fvp-r, GICV2)	1m 34s
build-arm (fvp-r, GICV2, CROSS_COMPILE=clang)	1m 31s
build-arm (fvp-r, GICV3)	1m 33s
build-arm (fvp-r, GICV3, CROSS_COMPILE=clang)	1m 35s
build-arm (fvp-r-aarch32, GICV2)	1m 35s
build-arm (fvp-r-aarch32, GICV2, CROSS_COMPILE=clang)	1m 33s
build-arm (fvp-r-aarch32, GICV3)	1m 35s
build-arm (fvp-r-aarch32, GICV3, CROSS_COMPILE=clang)	1m 37s
	25m 8s

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