hw/gpio/pl061: Clean up read/write offset handling logic

hw/gpio/pl061: Clean up read/write offset handling logic

Currently the pl061_read() and pl061_write() functions handle offsets
using a combination of three if() statements and a switch().  Clean
this up to use just a switch, using case ranges.

This requires that instead of catching accesses to the luminary-only
registers on a stock PL061 via a check on s->rsvd_start we use
an "is this luminary?" check in the cases for each luminary-only
register.

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