dm: regmap: Fix mask in regmap_update_bits()

dm: regmap: Fix mask in regmap_update_bits()

This function assumes that the 'val' parameter has no masked bits set.
This is not defined by the function prototype though. Fix the function to
mask the value and update the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>