efi_loader: AArch64: Run EFI payloads in EL2 if U-Boot runs in EL3

Some boards decided not to run ATF or other secure firmware in EL3, so
they instead run U-Boot there. The uEFI spec doesn't know what EL3 is
though - it only knows about EL2 and EL1. So if we see that we're running
in EL3, let's get into EL2 to make payloads happy.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: York Sun <york.sun@nxp.com>