uboot-imx

dm: Add support for register maps (regmap)

Add a simple implementaton of register maps, supporting only direct I/O
for now. This can be enhanced later to support buses which have registers,
such as I2C, SPI and PCI.

It allows drivers which can operate with multiple buses to avoid dealing
with the particulars of register access on that bus.

Signed-off-by: Simon Glass <sjg@chromium.org>