tests/qtest/boot-serial-test: Reorder pair of instructions in PL011 test

tests/qtest/boot-serial-test: Reorder pair of instructions in PL011 test

In the next commit we are going to use a different value
for the $w1 register, maintaining the same $x2 value. In
order to keep the next commit trivial to review, set $x2
before $w1.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>