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This patch removes the duplicate declaration of psci_cpu_on in psci.h
and moves the constants for the system level implementation of the
generic timer from arch_helpers.h to arch.h. All other architectural
constants are defined in arch.h so there is no need to add them to
arch_helpers.h

Change-Id: Ia8ad3f91854f7e57fce31873773eede55c384ff1