UPSTREAM: arch/riscv: Change all eret instructions to .word 0x30200073 (mret)

Using the opcode directly is necessary for the transition to the GCC
6.1.0 based toolchain, because the old toolchain only supports eret and
the new toolchain only supports mret.

BUG=None
BRANCH=None
TEST=None

Signed-off-by: Jonathan Neuschfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15290
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

Change-Id: I17e14d4793ae5259f7ce3ce0211cbb27305506cc
Reviewed-on: https://chromium-review.googlesource.com/361642
Commit-Ready: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/riscv/trap_util.S

@@ -124,9 +124,11 @@ trap_entry:
 supervisor_call_return:
   csrr a0, mscratch
   restore_regs
-  eret # go back into supervisor call
+  # go back into supervisor call
+  .word 0x30200073 # mret
   .global machine_call_return
 machine_call_return:
     csrr a0, mscratch
     restore_regs
-    eret # go back into machine call
+    # go back into machine call
+    .word 0x30200073 # mret