regmap: Support reading from specific range

It is useful to be able to treat the different ranges of a regmap
separately to be able to use distinct offset for them, but this is
currently not implemented in the regmap API.

To preserve backwards compatibility, add regmap_read_range and
regmap_write_range functions that take an additional parameter
'range_num' that identifies the range to operate on.

Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>