ports/zephyr: Add machine lightsleep.

Signed-off-by: Patrick Joy <[email protected]>

Author: ThinkTransit

docs/zephyr/quickref.rst

@@ -105,7 +105,7 @@ Hardware SPI is accessed via the :ref:`machine.SPI <machine.SPI>` class::
     spi.write_readinto(buf, buf)        # write buf to MOSI and read back into the buf
 
 Light-sleep mode
----------------
+----------------
 
 The following code can be used to sleep, reducing power consumption::