The regmap_read & regmap_write functions were previously declared in
regmap.h but not implemented anywhere. The regmap implementation &
commit message of 6f98b750 ("dm: Add support for register maps
(regmap)") indicate that only memory mapped accesses are supported for
now, so providing simple implementations of regmap_read & regmap_write
is trivial. The access size is presumed to be 4 bytes & endianness is
presumed native, which are the defaults for the regmap code in Linux.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Simon Glass <sjg@chromium.org>