powerpc/64: __ioremap_at clean up in the error case

__ioremap_at error handling is wonky, it requires caller to clean up
after it. Implement a helper that does the map and error cleanup and
remove the requirement from the caller.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>