usb: isp1760: move to regmap for register access

Rework access to registers and memory to use regmap framework.
No change in current feature or way of work is intended with this
change.

This will allow to reuse this driver with other IP of this family,
for example isp1763, with little changes and effort.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Link: https://lore.kernel.org/r/20210513084717.2487366-3-rui.silva@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>