hexagon: correct type on pgd copy

swapper_pg_dir is an array of pgd_t, not pgd_t*. This has no actual
effect since sizeof(pgd_t) == sizeof(pgd_t*), but unconfuses tools that
check types.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>