regmap: Allow buses to provide a custom update_bits() operation

Some buses provide a native _update_bits() operation which for uncached
registers is faster than doing a read/modify/write cycle as it is a
single bus transaction.  Add support for implementing this to regmap.

# gpg: Signature made Tue 06 Oct 2015 16:21:47 BST using RSA key ID 5D5487D0
# gpg: Oops: keyid_from_fingerprint: no pubkey
# gpg: Oops: keyid_from_fingerprint: no pubkey
# gpg: key 00000000 occurs more than once in the trustdb
# gpg: key 16005C11: no public key for trusted key - skipped
# gpg: key 16005C11 marked as ultimately trusted
# gpg: Good signature from "Mark Brown <broonie@sirena.org.uk>"
# gpg:                 aka "Mark Brown <broonie@debian.org>"
# gpg:                 aka "Mark Brown <broonie@kernel.org>"
# gpg:                 aka "Mark Brown <broonie@tardis.ed.ac.uk>"
# gpg:                 aka "Mark Brown <broonie@linaro.org>"
# gpg:                 aka "Mark Brown <Mark.Brown@linaro.org>"