clk: clk-gpio-gate: Fix active low

The active low flag in the DT cell is currently ignored.

This occurs because of_get_named_gpio_flags() does not apply the flags
to the underlying struct gpio_desc so the test in clk_register_gpio_gate()
was bogus.

Note that this patch changes the internal kernel API for
clk_register_gpio_gate() but there are currently no other users.

Signed-off-by: Martin Fuzzey <mfuzzey@parkeon.com>
Acked-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>