sparc: export openprom.h to userspace

sparc64 exports openprom.h to userspace so let sparc follow
the example.
As openprom.h pulled in another not-for-export vaddrs.h header
file it required a few changes to fix the build.

The definition af VMALLOC_* were moved to pgtable as this is
where sparc64 has them.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>