The SYS_CLOCK_HW_CYCLES_TO_NS macro was deprecated in zephyr commit
8892406c1de21bd5de5877f39099e3663a5f3af1. This commit updates MicroPython
to use the new k_cyc_to_ns_floor64 api and fix build warnings in the zephyr
port.
This change is compatible with Zephyr v2.1 and later.