fix: use 384KB TSRAM for `fvp-tbb-mbedtls-tsp-async`

This failure was missed when we updated the other failing builds to
384KB.

Change-Id: Ia83c1c792aee36f7eaa21fb891cdf3e846583dac
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/tf_config/fvp-tbb-mbedtls-tsp-async b/tf_config/fvp-tbb-mbedtls-tsp-async
index 92d1c16..c7072b4 100644
--- a/tf_config/fvp-tbb-mbedtls-tsp-async
+++ b/tf_config/fvp-tbb-mbedtls-tsp-async
@@ -6,3 +6,4 @@
 SPD=tspd
 TRUSTED_BOARD_BOOT=1
 TSP_INIT_ASYNC=1
+FVP_TRUSTED_SRAM_SIZE=384