optee-ftpm: Update to "main" branch

Since the github repository doesn't contain a "master" branch but has the
"main" branch now, this patch updates to the "main" branch so can
download the repository successfully.

CC: Ilias Apalodimas <ilias.apalodimas@linaro.org>
CC: Jerome Forissier <jerome.forissier@linaro.org>
CC: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-security/optee-ftpm/optee-ftpm_git.bb

@@ -19,7 +19,7 @@ DEPENDS = "python3-pyelftools-native optee-os-tadevkit python3-cryptography-nati
 
 FTPM_UUID="bc50d971-d4c9-42c4-82cb-343fb7f37896"
 
-SRC_URI = "gitsm://github.com/Microsoft/ms-tpm-20-ref;branch=master;protocol=https \
+SRC_URI = "gitsm://github.com/Microsoft/ms-tpm-20-ref;branch=main;protocol=https \
            file://0001-add-enum-to-ta-flags.patch"
 
 SRCREV = "d638536d0fe01acd5e39ffa1bd100b3da82d92c7"