fix/module: Add error messages to `static_assert` statements

This patch adds an error message to `static_assert` statements that
checks structure definitions mapped to real registers.
The LLVM compiler requires these error messages to be present.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: Iad80357bbc5a788e83cc07389be932f0c92d0bbc