For most systems, the ATA disk device driver must be bound into the base kernel because the root filesystem is on an ATA disk  and the kernel cannot mount the root filesystem, much less read any LKMs from it, without the ATA disk driver. But if you really want the device driver for your root filesystem to be an LKM, here's how to do it with Initrd:
"Initrd" is the name of the "initial ramdisk" feature of Linux. With this, you have your loader (probably LILO) load a filesystem into memory (as a ramdisk) before starting the kernel. When it starts the kernel, it tells it to mount the ramdisk as the root filesystem. You put the disk device driver for your real root filesystem and all the software you need to load it in that ramdisk filesystem. Your startup programs (which live in the ramdisk) eventually mount the real (disk) filesystem as the root filesystem. Note that a ramdisk doesn't require any device driver.
This does not free you, however, from having to bind into the base kernel 1) the filesystem driver for the filesystem in your ramdisk, and 2) the executable interpreter for the programs in the ramdisk.
You probably know this type of disk as "IDE". Strictly speaking, IDE is an incorrect appelation. IDE refers to the "Integrated Drive Electronics" which all modern disk drives, notably all SCSI disk drives, use. The first IDE drives in common usage were ATA, and the names kind of got confused. ATA, like SCSI, is a precise specification of electrical signals, commands, etc.