regmap: Add devm_regmap_init()

Most of new linux drivers are using managed-API to allocate resources. To
ease porting drivers from linux to U-Boot, introduce devm_regmap_init() as
a managed API to get a regmap from the device tree.

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