att6300p.cpp: Add support for 1.2MB drive (525hd)

[donohoe00]

The standard configurations for the 6300 Plus have drive A as a 1.2MB floppy, with either a hard drive or a 360K floppy as a secondary drive. One thing that's unique to the 6300 Plus is that port 0x65 is used for data rate control.

Since the FDC is on the motherboard, isa8_fdc_6300p_device is unique to the 6300 Plus... I'm wondering if it should go in a machine-specific file, instead of in fdc.cpp?