pinctrl: at91: replace clk_prepare + clk_enable by clk_prepare_enable

Replace the clk_prepare and clk_enable calls by a single
clk_prepare_enable call.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>