arm/trusted-firmware-a: add comment about location of deps

Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-bsp/trusted-firmware-a/trusted-firmware-a_2.10.4.bb

@@ -6,6 +6,7 @@ SRCBRANCH = "lts-v2.10"
 
 LIC_FILES_CHKSUM += "file://docs/license.rst;md5=b2c740efedc159745b9b31f88ff03dde"
 
+# in TF-A src, docs/getting_started/prerequisites.rst lists the expected version mbedtls
 # mbedtls-3.4.1
 SRC_URI_MBEDTLS = "git://github.com/ARMmbed/mbedtls.git;name=mbedtls;protocol=https;destsuffix=git/mbedtls;branch=master"
 SRCREV_mbedtls = "72718dd87e087215ce9155a826ee5a66cfbe9631"