iio: adc: ad7887: invert/rework external ref logic

This change inverts/reworks the logic to use an external reference via a
provided regulator.

Now the driver tries to obtain a regulator. If one is found, then it is
used. The rest of the driver logic already checks if there is a non-NULL
reference to a regulator, so it should be fine.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201002082723.184810-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 149

Based on 1 normalized pattern(s):

licensed under the gpl 2 or later

extracted by the scancode license scanner the SPDX license identifier

GPL-2.0-or-later

has been chosen to replace the boilerplate/reference in 82 file(s).

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Richard Fontana <rfontana@redhat.com>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190524100845.150836982@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

staging:iio: Move the ad7887 driver out of staging

The driver does not expose any custom API to userspace and none of the standard
static code checker tools report any issues, so move it out of staging.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>