ARM: dts: dra7: Use clksel binding for CM_CLKSEL_DPLL_GPU

ARM: dts: dra7: Use clksel binding for CM_CLKSEL_DPLL_GPU

With the clkcsel binding we can drop the custom ti,bit-shift devicetree
property in favor of the standard reg property and reduce the number of
clocks to update for the make W-1 dtbs warnings.

Let's also add a comment for the clocksel clock that matches the
documentation.

Signed-off-by: Tony Lindgren <tony@atomide.com>