[PATCHv6] staging/iio/adc: change the MXS touchscreen driver implementation

[Juergen Beisert <jbe@pengutronix.de>]

The following series replaces the current busy loop touchscreen implementation
for i.MX28/i.MX23 SoCs by a fully interrupt driven implementation.

Since i.MX23 and i.MX28 silicon differs, the existing implementation can
be used for the i.MX28 SoC only.

The first patch adds proper clock handling. Various platforms seems to disable
the internal 2 kHz clock which is used by the LRADC delay units.

The next two patches of this series move the i.MX28 specific definitions
out of the way. The forth patch simplifies the register access to make it easier
to add the i.MX23 support. Then the i.MX23 specific definitions are added, also
the code to distinguish both SoCs at run-time.
Up to here the existing touchscreen driver will now run on an i.MX23 Soc as well.

When these i.MX SoCs are running from battery it seems not to be a good idea to
run a busy loop to detect touches and their location. The 6th patch adds a
fully interrupt driven implementation which makes use of the built-in delay
and multiple sample features of the touchscreen controller. This will reduce
the interrupt load to a minimum.

The remaining patches in this series just removes the existing busy loop
implementation, add a proposal for devicetree binding and a reminder what has
still to be done with the LRADC driver.

Changes since v5:

- add missing clock handling which prevents the delay units from work (this
  should make it work on the MX28EVK and M28EVK as well)

Changes since v4:

- honor Jonathan's comments about function names
- honor Dmitry's comments about workqueue canceling and interrupts
- adding devicetree bindings proposal

Changes since v3:

- split adding register access functions and i.MX23 support into two patches

Changes since v2:

- useless debug output removed

Changes since v1:

- adding register access functions to make the existing code more readable
- adding some functions to distinguish the SoCs at run-time to avoid if-else
  contructs whenever differences in the register layout between i.MX23 and
  i.MX28 must be handled

Comments are welcome.

Juergen