regmap: Add better support for devices without readback support

Currently regmap requires that a reg_read callback is supplied, otherwise a
warning is emitted each time regmap_read() is called. This means a device
or bus without readback support needs to supply dummy reg_read callback.
Apart from that regmap_read() will still work fine if a cache is used.

Remove the warning and let regmap_readable() return false if not reg_read
callback is supplied. This means a device no longer has to supply a dummy
callback if it does not support readback and it also doesn't have to have a
readable_reg callback that always returns false since this is now implicit.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>