test: dm: Add tests for regmap managed API and regmap fields

The tests rely on a dummy driver to allocate and initialize the regmaps
and the regmap fields using the managed API. The first test checks if
the regmap config fields like width, reg_offset_shift, range specifiers,
etc work. The second test checks if regmap fields behave properly (mask
and shift are ok) by peeking into the regmap.

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