net: ipa: generalize register field functions

Rename functions related to register fields so they don't appear to
be IPA-specific, and move their definitions into "reg.h":
    ipa_reg_fmask()	-> reg_fmask()
    ipa_reg_bit()	-> reg_bit()
    ipa_reg_field_max()	-> reg_field_max()
    ipa_reg_encode()	-> reg_encode()
    ipa_reg_decode()	-> reg_decode()

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