Have a google about for the source of the A1200 IDE driver for Linux, they also mention the registers for controlling the PCMCIA slot too IIRC.
Kernel list archive mentions getting an E-Matrix A530 working, looks like that behaves just like a Gayle controlled IDE interface.
BTW, A1200 40.70
AFAIK, the last A1200 ROM was 40.68, 40.70 was released for the A3000 and A4000, and just had an updated scsi.device compared to 40.68.
Dunno what machine you're trying to make this on, but you do need at least an 020 if you're using an A1200 ROM