regmap: Add API regmap_init_mem_index()

In device nodes with more than one entry in the reg property,
it is sometimes useful to regmap only of the entries. Add an
API regmap_init_mem_index() to facilitate this.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>