Third set of IIO new device support, cleanups and features for the 4.19 cycle.

This is a somewhat sneaky late pull request given Linus has announced
a likely 1 week delay.  If it is too late I'll resend after the merge
window.  I merged in the fixes branch as those are primarily around
things queued for the merge window.

Particulary good to see the output of our Himanshu Jha, a GSOC student
who developed the bme680 driver.

New device support:
* bme680 gas sensor (with temperature, humidity and pressure)
  - new driver to support this device
* vcn4000
  - support vcnl4200 (lots of rework to allow this)
  - ids added for VCNL4010 and VCNL4020 sensors

New features:
* ad9523
  - support the various external signal options via gpios.

Cleanups and fixes
* ad_sigma_delta
  - unsigned long for a timeout.
* ad9523
  - fix a wrong return value that was indicating successful write failed
    and might lead to an infinite loop.
* si1133
  - fix an impossible test.
  - fix an uninitialsed variable by reading from the device in all paths.
* xilinx xadc
  - check for return values in clk related functions
  - limit the pcap clock frequency to supported ranges.
  - stash the irq to avoid calling platform_get_irq from the remove path.
  - ensure the irq is actually requested before we enable the hardware to
    output it.
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