hw/gpio/pl061: Add tracepoints for register read and write

hw/gpio/pl061: Add tracepoints for register read and write

Add tracepoints for reads and writes to the PL061 registers. This requires
restructuring pl061_read() to only return after the tracepoint, rather
than having lots of early-returns.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>