ARM: dts: imx6sll: add gpio clocks

i.MX6SLL has GPIO clock gates in CCM CCGR, add
clock property for GPIO driver to make sure all
GPIO banks work as expected.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>