clk: at91: fix h32mx prototype inclusion in pmc header

Trivial fix that prevents to compile this pmc clock driver if h32mx clock is
present but smd clock isn't.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Fixes: bcc5fd49 ("clk: at91: add a driver for the h32mx clock")
Cc: <stable@vger.kernel.org> # 3.18+