The Device Driver Programming manual, pub number 0890425-020, is a
comprehensive guide on how to write your own kernel level device driver
and integrate it into the kernel.  There is a short information gap between
the chapters on writing the driver (and how to multithread it and support
DMA) and integrating it into the system which explains the necessary C
compiler options and how to relocatably link your driver before including
it in the kernel.

Here it is in a nutshell:

To compile your driver, you must use the following C compiler options:

-c -O -Xa -F -F1 -Zelf -Zpic -Zlink=dynamic
-Qchars_signed -Qno_float_varargs -Qno_float_reg_agg_move -Qtarget=ppc604
-D_PPC -D_NH -D_KERNEL -D_KERNEL_HEADERS -DSTATIC='' -DNO_RDMA
-D_LARGEFILE64_SOURCE

For details about what each of the (non -D) options mean, consult the C
compiler man page cc(1), the Concurrent C Reference Manual, and Compilation
Systems Volumes 1 and 2.

After generating the .o file, you must link the .o file as a relocatable
object (see ld(1)).  The resulting file will be the Driver.o that's linked
into the kernel using the idtools (see Chapter 9 of the Device Driver
Programming manual).

For example, say you created a driver for a serial I/O board.  You name the
driver source sio.c, with include file sio.h and a makefile, sio.mk.
Optionally, you could make a subdirectory, sio.cf, where the relocatably
linked Driver.o would be stored.

The makefile would contain a line to compile sio.c with the above options
to generate sio.o.  After compilation, you would link sio.o as follows:

ld -r -o sio.cf/Driver.o sio.o

The Driver.o created would be the one integrated into the kernel using the
idtools and procedures for creating the other components of the driver
package, as described in the Device Driver Programming manual, Chapter 9.