Add driver for veml6030 ambient light sensor

[Rishi Gupta]

The veml6030 is an ambient light sensor from vishay and
is a different hardware from an existing hardware for which
driver currently exist, therefore this driver submission.

* All features; ALS, white channel & power management is
  supported.

* All configurable parameters are supported through standard
  iio sysfs entries. User space can get valid values of any
  parameter (xx_available) and then can write to appropriate
  sysfs entry.

* User space can get ALS & White channel readings through RAW
  IIO interface.

* IIO events are used to notify application whenever threshold
  is crossed. This uses IRQ pin of veml6030.

* Some registers in veml6030 are read only. For these registers
  read callback returns error to user space.

There are 2 patches for this submission:
[PATCH 1/2] iio: light: add driver for veml6030 ambient light sensor
[PATCH 2/2] dt-bindings: iio: light: add veml6030 ALS bindings

Regards,
Rishi