xtensa: userspace: use ADDX4 to calculate syscall table index

When looking for jump address in the syscall table, we need to
multiply the syscall ID by 4 before adding the address offset
of the beginning of the table. This is due to the jump address
being 32-bit (4 bytes). Instead of using two instructions to
shift the ID by 4 first and then the addition, we can use one
ADDX4 instruction to achieve the same result.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/xtensa/core/userspace.S

@@ -119,8 +119,7 @@ _not_checking_user_context:
 _id_ok:
 	/* Find the function handler for the given syscall id. */
 	movi a3, _k_syscall_table
-	slli a2, a2, 2
-	add a2, a2, a3
+	addx4 a2, a2, a3
 	l32i a2, a2, 0
 
 #if XCHAL_HAVE_THREADPTR