clk: imx6ul: add GPIO clock gates

i.MX6UL has GPIO clock gates in CCM CCGR,
add them into clock tree for clock management.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>