regmap: Allow specifying read/write width

Right now, regmap_read() and regmap_write() read/write a 32-bit value
only. To write other lengths, regmap_raw_read() and regmap_raw_write()
need to be used.

This means that any driver ported from Linux that relies on
regmap_{read,write}() to know the size already has to be updated at each
callsite. This makes the port harder to maintain.

So, allow specifying the read/write width to make it easier to port the
drivers, since now the only change needed is when initializing the
regmap.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>