net: ipa: fix up IPA register comments

Revise or add comments in "ipa_reg.h" for to provide more
information, and to improve clarity and consistency.
  - Always provide a comment to define when a register or field is
    supported (or not) for certain versions of IPA hardware.
  - Try to be specific about *which* or *how many* definitions
    a comment refers to.
  - Move comments stating that ipa->available defines the valid
    bits in various registers *above* the register offset
    definition, to avoid some checkpatch.pl warnings.
No code is changed by this patch.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>