media: platform: place stm32/ and sti/ under st/ dir

As the end goal is to have platform drivers split by vendor,
move both stm32/ and sti/ for them to be inside st/ directory.

Acked-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>