Use the array of register descriptors assigned at initialization
time to determine the offset (and where used, stride) for IPA
registers.  Issue a warning if an offset is requested for a register
that's not valid for the current system.

Remove all IPE_REG_*_OFFSET macros, as well as inline static
functions that returned register offsets.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>