arch/riscv/trap_util.S: Use "li" pseudo-instruction to load a constant

Change-Id: I9759771fa6fc708d7d97509c5f5e0cefb8ab4c96
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/14962
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/trap_util.S

@@ -108,7 +108,7 @@
 supervisor_trap_entry:
     csrw mscratch, sp
     # load in the top of the machine stack
-    la sp, 0x80FFF0 - 64
+    li sp, 0x80FFF0 - 64
     1:addi sp,sp,-320
     save_tf
     move  a0,sp