docs(imx8m): update build support for imx8mq

Due to the small OCRAM space used for TF-A, we will
meet imx8mq build failure caused by too small RAM size.
We CANNOT support it in TF-A CI. It does NOT mean that
imx8mq will be dropped by NXP. NXP will still actively
maintain it in NXP official release.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Iad726ffbc4eedc5f6770612bb9750986b9324ae9

docs/plat/imx8m.rst

@@ -6,6 +6,9 @@ cores provide high-performance computing, power efficiency, enhanced system
 reliability and embedded security needed to drive the growth of fast-growing
 edge node computing, streaming multimedia, and machine learning applications.
 
+imx8mq is dropped in TF-A CI build due to the small OCRAM size, but still actively
+maintained in NXP official release.
+
 Boot Sequence
 -------------