Don't use empty structs when T/L conversion and/or MERAM are disabled

Empty struct definitions are not really allowed in C and can result in
undefined, compiler dependent behavior. Add a dummy member instead.
Only define libshmeram dependent shvpu_meram_t when needed

When using the kernel PMB device for T/L conversion, we don't need to
have definitions for the members of shvpu_meram_t.  As such, we shouldn't
be looking for <meram/meram.h>, which doesn't need to exist in this
configuration.