clk: imx6sll: add GPIO LPCGs

According to Reference Manual Rev.0, 06/2017, there are GPIO LPCGs
defined in CCM CCGRs, add them into clock tree.

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>