misc/byteorder.h: Add a function to store a 64-bit int in LE

sys_put_le64 stores a 64-bit unsigned integer into an
arbitrary memory location and is alignment-safe for any
architecture.

Change-Id: Ifd9871a509b9cab05a59d81f4917c68dda3cc824
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>