How do I add a third ATA controller to 4.8?

[Paul Hoffman]

Greetings again. I have a Dell 600SC that has three ATA controllers 
on the motherboard, and I'm running 4.8 quite happily on it. The 
GENERIC kernel has the following:

device      ata0    at isa? port IO_WD1 irq 14
device      ata1    at isa? port IO_WD2 irq 15

Since I didn't know the irq used by the third controller (it's not 
listed in their documentation, of course!), I blithely tried adding:

device      ata2    at isa? port IO_WD3

However, when rebuilding this kernel, I get the following error:

cc -c -O -pipe  -Wall -Wredundant-decls -Wnested-externs 
-Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith -Winline 
-Wcast-qual  -fformat-extensions -ansi  -nostdinc -I- -I. -I../.. 
-I../../../include -I../../contrib/ipfilter  -D_KERNEL -include 
opt_global.h  -mpreferred-stack-boundary=2  ioconf.c
ioconf.c:48: `IO_WD3' undeclared here (not in a function)
ioconf.c:48: initializer element is not constant
ioconf.c:48: (near initialization for `ata2_resources[1].u.longval')
*** Error code 1

So, what am I supposed to add to the kernel to add this third controller?

--Paul Hoffman