iio:adc: add iio driver for Palmas (twl6035/7) gpadc

iio:adc: add iio driver for Palmas (twl6035/7) gpadc

This driver code was found as:

https://android.googlesource.com/kernel/tegra/+/aaabb2e045f31e5a970109ffdaae900dd403d17e/drivers/staging/iio/adc

Fixed various compilation issues and test this driver on omap5 evm.

Signed-off-by: Pradeep Goudagunta <pgoudagunta@nvidia.com>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>