UBUNTU:SAUCE: iio: st_sensors: support active-low interrupts

UBUNTU:SAUCE: iio: st_sensors: support active-low interrupts

BugLink: http://bugs.launchpad.net/bugs/1650112
Most ST MEMS Sensors that support interrupts can also handle sending
an active low interrupt, i.e. going from high to low on data ready
(or other interrupt) and thus triggering on a falling edge to the
interrupt controller.

Set up logic to inspect the interrupt line we get for a sensor: if
it is triggering on rising edge, leave everything alone, but if it
triggers on falling edges, set up active low, and if unsupported
configurations appear: warn with errors and reconfigure the interrupt
to a rising edge, which all interrupt generating sensors support.

Create a local header for st_sensors_core.h to share functions
between the sensor core and the trigger setup code.

Cc: Giuseppe Barba <giuseppe.barba@st.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
(back ported from commit a9fd053b56c6bb14972ab7a19da0b575fe4c5d66)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Conflicts:
	drivers/iio/common/st_sensors/st_sensors_core.c
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>