hw/arm/palm.c: Encapsulate misc GPIO handling in a device

hw/arm/palm.c: Encapsulate misc GPIO handling in a device

Replace the free-floating set of IRQs and palmte_onoff_gpios()
function with a simple QOM device that encapsulates this
behaviour.

This fixes Coverity issue CID 1421944, which points out that
the memory returned by qemu_allocate_irqs() is leaked.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200628214230.2592-3-peter.maydell@linaro.org