net: ipa: add per-version IPA register definition files

Create a new subdirectory "reg", which contains a register
definition file for each supported version of IPA.  Each register
definition contains the register's offset, and for parameterized
registers, the stride (distance between consecutive instances of the
register).  Finally, it includes an all-caps printable register name.

In these files, each IPA version defines an array of IPA register
definition pointers, with unsupported registers defined with a null
pointer.  The array is indexed by the ipa_reg_id enumerated type.

At initialization time, the appropriate register definition array to
use is selected based on the IPA version, and assigned to a new
"regs" field in the IPA structure.

Extend ipa_reg_valid() so it fails if a valid register is not
defined.

This patch simply puts this infrastructure in place; the next will
use it.

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