hw/i2c/aspeed: support to set the different memory size

hw/i2c/aspeed: support to set the different memory size

According to the datasheet of ASPEED SOCs,
an I2C controller owns 8KB of register space for AST2700,
owns 4KB of register space for AST2600, AST2500 and AST2400,
and owns 64KB of register space for AST1030.

It set the memory region size 4KB by default and it does not compatible
register space for AST2700.

Introduce a new class attribute to set the I2C controller memory size
for different ASPEED SOCs.

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