docs(imx8m): update for measured boot for imx8mm

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Change-Id: Ib313dc1ffac2fc5d04e0779c9f059236a71e65e7
diff --git a/docs/plat/imx8m.rst b/docs/plat/imx8m.rst
index 0fe15c9..101d52b 100644
--- a/docs/plat/imx8m.rst
+++ b/docs/plat/imx8m.rst
@@ -60,3 +60,11 @@
 All of the BL3x will be put in the FIP image. BL2 will verify them.
 In U-boot we turn on the UEFI secure boot features so it can verify
 grub. And we use grub to verify linux kernel.
+
+Measured Boot
+-------------
+
+When setting MEASURED_BOOT=1 on imx8mm we can let TF-A generate event logs
+with a DTB overlay. The overlay will be put at PLAT_IMX8M_DTO_BASE with
+maximum size PLAT_IMX8M_DTO_MAX_SIZE. Then in U-boot we can apply the DTB
+overlay and let U-boot to parse the event log and update the PCRs.