hw/watchdog/wdt_aspeed: Rename MMIO region size as 'iosize'

hw/watchdog/wdt_aspeed: Rename MMIO region size as 'iosize'

Avoid confusing two different things:
- the WDT I/O region size ('iosize')
- at which offset the SoC map the WDT ('offset')
While it is often the same, we can map smaller region sizes
at larger offsets.

Here we are interested in the I/O region size, so rename as
'iosize'.

Reviewed-by: Peter Delevoryas <peter@pjd.dev>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
[ clg: Introduced temporary wdt_offset variable ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>