clk: imx6: fix i.MX6DL clock tree to reflect reality

The current clock tree only implements the minimal set of differences
between the i.MX6Q and the i.MX6DL, but that doesn't really reflect
reality.

Apply the following fixes to match the RM:
- DL has no GPU3D_SHADER_SEL/PODF, the shader domain is clocked by
  GPU3D_CORE
- GPU3D_SHADER_SEL/PODF has been repurposed as GPU2D_CORE_SEL/PODF
- GPU2D_CORE_SEL/PODF has been repurposed as MLB_SEL/PODF

Cc: stable@vger.kernel.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>