Squashed 'connectivity/drivers/802.15.4_RF/stm-s2lp-rf-driver/' changes from 9441d65aab..063b1af74e

063b1af74e Convert files to unix format
e2b972d54b Merge pull request #72 from ARMmbed/sync_with_mbedos
05b490966f Flag chrono functions to support Mbed OS 5.15
b5296f000e (via Mbed-OS)Use std::chrono based functions
0eaa99b633 Fix error found by coverity (#71)

git-subtree-dir: connectivity/drivers/802.15.4_RF/stm-s2lp-rf-driver
git-subtree-split: 063b1af74e4eca7002d3b1c973a99d08feee37f8

Author: Arto Kinnunen