efi: add comment for efi_system_table and efi_configuration_table

This commit adds the comment for efi_system_table and
efi_configuration_table structure.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>