clk: at91: fix num_parents test in at91sam9260 slow clk implementation

The slow clk block provided by at91sam9260 and derived SoCs should always
have 2 parents.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>