hw/i2c/aspeed: Support discontinuous register memory region of I2C bus

hw/i2c/aspeed: Support discontinuous register memory region of I2C bus

It only support continuous register memory region for all I2C bus.
However, the register address of all I2c bus are discontinuous
for AST2700.

Ex: the register address of I2C bus for ast2700 as following.
0x100 - 0x17F: Device 0
0x200 - 0x27F: Device 1
0x300 - 0x37F: Device 2
0x400 - 0x47F: Device 3
0x500 - 0x57F: Device 4
0x600 - 0x67F: Device 5
0x700 - 0x77F: Device 6
0x800 - 0x87F: Device 7
0x900 - 0x97F: Device 8
0xA00 - 0xA7F: Device 9
0xB00 - 0xB7F: Device 10
0xC00 - 0xC7F: Device 11
0xD00 - 0xD7F: Device 12
0xE00 - 0xE7F: Device 13
0xF00 – 0xF7F: Device 14
0x1000 – 0x107F: Device 15

Introduce a new class attribute to make user set each I2C bus gap size.
Update formula to create all I2C bus register memory regions.

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