hw/intc: clean-up access to GIC multi-byte registers

hw/intc: clean-up access to GIC multi-byte registers

gic_dist_readb was returning a word value which just happened to work
as a result of the way we OR the data together. Lets fix it so only
the explicit byte is returned for each part of GICD_TYPER. I've
changed the return type to uint8_t although the overflow is only
detected with an explicit -Wconversion.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>