iio: add vcnl4000 combined ALS and proximity sensor

minimal driver, no IR current control and proximity/event
handling yet

v5:
* checkpatch warnings
* increase msleep() to 20 ms when waiting for data ready as
  measurement/conversion can take up to 100 ms, 1 ms is too short

v4 (address comments by Jonathan Cameron)
* remove SENSORS_ prefix in Kconfig
* change from IIO_INTENSITY to IIO_LIGHT
* move from staging

v3 (address comments by Shubhrajyoti Datta)
* cleanup Kconfig entry
* call I2C read/write functions directly

v2 (address comments by Lars-Peter Clausen and Jonathan Cameron)
* unify code for reading PS and AL data into
  parameterized _measure() function
* limit wait for data to become ready within 20 tries
* drop IIO_LIGHT channel, add SCALE to IIO_INTENSITY
* drop extra string arguments used for logging purpose only

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>