Documentation: Add ISA bus driver documentation

This is a verbatim copy of the original commit message of the initial
commit of the ISA bus driver authored by Rene Herman. Descriptions of
the module_isa_driver macro and max_num_isa_dev macro are provided at
the end.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>