regmap: Implement regmap_multi_reg_read()

Merge series from Guenter Roeck <linux@roeck-us.net>:

regmap_multi_reg_read() is similar to regmap_bilk_read() but reads from
an array of non-sequential registers. It is helpful if multiple non-
sequential registers need to be read in a single operation which would
otherwise have to be mutex protected.

The name of the new function was chosen to match the existing function
regmap_multi_reg_write().