regmap: Define regmap_{get,set}

It would be convenient if one could use the regmap API in conjunction
with register maps defined as structs (i.e. structs that directly mirror
the memory layout of the registers in question). A similar approach was
planned with the regmap_write32/regmap_read32 macros, but was never
used.

Hence, implement regmap_set/regmap_range_set and
regmap_get/regmap_range_get macros, which, given a register map, a
struct describing the layout of the register map, and a member name
automatically produce regmap_read/regmap_write calls that access the
specified member in the register map.

Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>