net: ipa: store BCR register values in config data

The backward compatibility register value is a platform-specific
property that is not stored in the platform data.  Create a data
field where this can be represented, and get rid ipa_reg_bcr_val().

This register is not present starting with IPA v4.5.

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