staging: iio: light: isl29018: add support for isl29023 and isl29035

Intersil chips ISL29018, ISL29023 and ISL29035 are very similar. They're
all ambience light sensors. The ISL29018, however, is also a proximity
sensor. The registers are similar too:

-------------+----------+----------
AVAILABLE IN | ADDR REG | NAME
   290xx     |          |
-------------+----------+----------
     18/23/35|       00h| COMMANDI
     18/23/35|       01h| COMMANDII (B4-7 are used only in 29018 for proximity)
     18/23/35|       02h| DATALSB
     18/23/35|       03h| DATAMSB
     18/23/35|       04h| INT_LT_LSB
     18/23/35|       05h| INT_LT_MSB
     18/23/35|       06h| INT_HT_LSB
     18/23/35|       07h| INT_HT_MSB
        18/23|       08h| TEST
           35|       0Fh| ID
-------------+----------+-----------

So, this patch will add support for ISL29023 and ISL29035 to the
existing isl29018 driver. Since these 2 chips don't have proximity
detection, the proximity sysfs attribute is not needed.

Also, for ISL29035, since it has an ID register, make use of it in order
to properly detect the chip and clear the brownout bit.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>