hw/gpio/aspeed: Support to set the different memory size

hw/gpio/aspeed: Support to set the different memory size

According to the datasheet of ASPEED SOCs, a GPIO controller owns 4KB of
register space for AST2700, AST2500, AST2400 and AST1030; owns 2KB of
register space for AST2600 1.8v and owns 2KB of register space for
AST2600 3.3v.

It set the memory region size 2KB by default and it does not compatible
register space for AST2700.

Introduce a new class attribute to set the GPIO controller memory size
for different ASPEED SOCs.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>