buildroot: Add optee configuration file for RISC-V 64-bit system

Add new file to specify the target architecture to run OP-TEE.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/br-ext/configs/optee_riscv64 b/br-ext/configs/optee_riscv64
new file mode 100755
index 0000000..9a55a03
--- /dev/null
+++ b/br-ext/configs/optee_riscv64
@@ -0,0 +1,2 @@
+BR2_riscv=y
+BR2_RISCV_64=y