Documentation: dt: Remove clock gates IDs list for Allwinner SoCs

That documentation was mostly useful when we didn't have any
documentation for those SoCs, which is not the case anymore. Remove
this, since it should live in the DT anyway.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Mark Rutland <mark.rutland@arm.com>