drivers: iio: bmi160: Enabled the buffering support for BMI160 Sensor

This patch allows to buffer bmi160 accel and gyro samples from driver
up to time till user space services are up and running
(or upto nth second). Buffered samples can be retrived at any time via
input subsystem by writing to below sysfs entry.

echo 1 > /sys/bus/iio/devices/iio:device0/read_acc_boot_sample
echo 1 > /sys/bus/iio/devices/iio:device0/read_gyro_boot_sample

Use config option to enable buffering feature.

CRs-Fixed: 2012545
Change-Id: I8f2cb23a844325b67c768d8bd413874158a94026
Signed-off-by: puneet <puneet@codeaurora.org>