tty/serial: atmel: remove cache when unnecessary

struct cache is only used in suspend/resume. Exclude it when PM is not
selected.

Suggested-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>