Add missing license and copyright headers

The following files didn't have any copyright or license headers on them
when they got contributed.  So add the SPDX Apache license and
appropriate copyright info:

boards/arm/stm32l476g_disco/pinmux.c
samples/basic/threads/src/main.c
tests/net/socket/tcp/src/main.c

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>