Implemented GSI allocator and system allocator for AArch64.
Renamed some layout definitions to align more code between architectures.
Signed-off-by: Michael Zhao <michael.zhao@arm.com>
GSI (Global System Interrupt) is an extension of just a linear array of
IRQs. It takes IOAPICs into account for example.
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>