include/efi_api.h: Add more detailed API definitions

The EFI API header is great, but missing a good chunk of function prototype,
GUID defines and enum declarations.

This patch extends it to cover more of the EFI API. It's still not 100%
complete, but sufficient enough for our EFI payload interface.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>