net: ipa: generalize register offset functions

Rename ipa_reg_offset() to be reg_offset() and move its definition
to "reg.h".  Rename ipa_reg_n_offset() to be reg_n_offset() also.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>