clk: sunxi: drop CLK_IGNORE_UNUSED

This flag was in place to prevent important clocks from getting gated
while they had no users. Now that the UART driver supports clocks
properly, we can drop this.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>