media: admin-guide: add SPI cards to platform drivers

The SPI cards are also under the concept of "platform" drivers
as defined at the section introduction.

So, add a SPI card list there.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/cardlist.rst

@@ -352,6 +352,15 @@ xilinx-video       Xilinx Video IP (EXPERIMENTAL)
 xilinx-vtc         Xilinx Video Timing Controller
 =================  ============================================================
 
+MMC/SDIO DVB adapters
+---------------------
+
+=======  ===========================================
+Driver   Name
+=======  ===========================================
+smssdio  Siano SMS1xxx based MDTV via SDIO interface
+=======  ===========================================
+
 I²C drivers
 ===========