IT87: Add ability to select between chips on GIGABYTE DualBIOS boards.

Thanks to Vadim Girlin for finding out how to do that
The patent describing the overall implementation of DualBIOS can be found
here: http://www.google.com/patents/US6892323
This is known to work on GA-MA770-UD3, GA-B75M-D3V and GA-H61M-S1 (only
M_BIOS is populated).

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Signed-off-by: Damien Zammit <damien@zamaudio.com>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>