arm64: Allow EFI payload code to take exceptions

There are 2 ways an EFI payload could return into u-boot:

  - Callback function
  - Exception

While in EFI payload mode, x18 is owned by the payload and may not contain
a valid pointer to gd, so we need to fix it up. We do that properly for the
payload to callback path already.

This patch also adds gd pointer restoral for the exception path.

Signed-off-by: Alexander Graf <agraf@suse.de>