iio: pressure: mpl115: Implementing low power mode by shutdown gpio

iio: pressure: mpl115: Implementing low power mode by shutdown gpio

MPL115 supports shutdown gpio which can be used to set the state
to low power mode. Power from all internal circuits and
registers is removed. This is done by pulling the SHDN pin to low.
This patch enables runtime PM on MPL115 to increase power savings.

According to spec., a wakeup time period of ~5 ms exists between
waking up and actually communicating with the device. This is
implemented using sleep delay.

Signed-off-by: Rajat Khandelwal <rajat.khandelwal@linux.intel.com>
Link: https://lore.kernel.org/r/20221010173720.568916-1-rajat.khandelwal@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>