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Add basic SDEI support, implementing the software event 0 only for now.
This already allows hypervisors like Jailhouse to use SDEI for internal
signaling while passing the GICC through to the guest (see also IMX8).

With SDEI on, we overrun the SRAM and need to stay in DRAM. So keep SDEI
off by default.
Co-developed-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Change-Id: Ic0d71b4ef0978c0a34393f4e3530ed1e24a39ca2