clk: at91: remove the useless CLK_IGNORE_UNUSED flag

The CLK_IGNORE_UNUSED flag was added on all the system clocks because of the
ddrck. Now that it is handled by the ram controller driver, we can drop it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>