Need APIs to add regmap in dev

[Laxman Dewangan <ldewangan@nvidia.com>]

Hi Mark,
when working on unification of Max77620, MAX77686 RTC driver, I came to 
need of APIs from regmap to add the device  as devres.

int dev_add_regmap(dev, regmap);

This will add the regmap in the dev resource of device. This will help 
to get the regmap handle from device using dev_get_regmap(). This APIs 
will be different from the regmap_attach_dev() where map->dev is moved 
to new device.

The requirement came from multiple i2c slave address device like MAX77620.
MAX77620 has 2 i2c address 0x3C and 0x68. 0x68 is for RTC IP block.
we register device with 0x3C and create dummy i2c client for 0x68 inside 
driver.
We create two regmap, one from 0x3C i2c client as rmap_0x3c and other 
from 0x68 i2c dummy client rmap_0x68.

Now in RTC driver(mfd device), we have the parent device as dev_0x3c.
When we try to get the regmap handle, we only get one regmap per device 
as we register regmap with different devices.

If I had above API then I can add the rmap_0x68 in the dev_0x3c as 
dev_res. By this I can have two regmap list on dev_0x3c and get rmap 
handle by dev_get_regmap(dev_0x3c, "pmic-slave") and 
dev_get_regmap(dev_0x3c, "rtc-slave");


I like to know your comment on this approach to add the regmap on 
different device resource.

Thanks,
Laxman