Always use eMMC RPMB support when adding stmm

U-Boot needs to support RPMB for the stmm backend to be able to use it
for storing variables.

Signed-off-by: Grant Likely <grant.likely@arm.com>

stmm.config

@@ -2,3 +2,4 @@ CONFIG_TEE=y
 CONFIG_OPTEE=y
 CONFIG_OPTEE_TA_AVB=y
 CONFIG_EFI_MM_COMM_TEE=y
+CONFIG_SUPPORT_EMMC_RPMB=y