it87spi.c: Added ability for IT8728F to flash dualbioses on Gigabyte boards.

For Gigabyte B75M-D3V mainboard, this enables the internal programmer
to select between the two bios flash chips and flash them independently.

Usage:
	flashrom -p internal:gbdualindex=0
	flashrom -p internal:gbdualindex=1

Signed-off-by: Damien Zammit <damien@zamaudio.com>