Update the header comments in .hx files that mention STEXI/ETEXI
markup; this is now SRST/ERST as all these files have been
converted to rST.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200306171749.10756-3-peter.maydell@linaro.org