refactor(lib/smc): allow smc-rmi.h to be included in asm

This change exposes the structure definitions only if
__ASSEMBLER__ is not defined there by allowing this header
to be included in assembler source files.

Signed-off-by: Yousuf A <yousuf.sait@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: Ie45b22f150829af01905db4b51ead48a5bfb090e