net: ipa: introduce IPA register IDs

Create a new ipa_reg_id enumerated type, which identifies each IPA
register with a symbolic identifier.  Use short names, but in some
cases (such as "BCR") add "IPA_" to the name to help avoid name
conflicts.

Create two functions that indicate register validity.  The first
concisely indicates whether a register is valid for a given version
of IPA, and if so, whether it is defined.  The second indicates
whether a register is valid for TX or RX endpoints.

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