regmap: Add raw read/write functions

The regmap functions currently assume that all register map accesses
have a data width of 32 bits, but there are maps that have different
widths.

To rectify this, implement the regmap_raw_read and regmap_raw_write
functions from the Linux kernel API that specify the width of a desired
read or write operation on a regmap.

Implement the regmap_read and regmap_write functions using these raw
functions in a backwards-compatible manner.

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