hw/gpio/aspeed: Don't let guests modify input pins

hw/gpio/aspeed: Don't let guests modify input pins

Up until now, guests could modify input pins by overwriting the data
value register. The guest OS should only be allowed to modify output pin
values, and the QOM property setter should only be permitted to modify
input pins.

This change also updates the gpio input pin test to match this
expectation.

Andrew suggested this particularly refactoring here:

    https://lore.kernel.org/qemu-devel/23523aa1-ba81-412b-92cc-8174faba3612@www.fastmail.com/

Suggested-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Peter Delevoryas <peter@pjd.dev>
Fixes: 4b7f956862dc ("hw/gpio: Add basic Aspeed GPIO model for AST2400 and AST2500")
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220712023219.41065-3-peter@pjd.dev>
Signed-off-by: Cédric Le Goater <clg@kaod.org>