bus: simple-pm-bus: convert bool SIMPLE_PM_BUS to tristate

The Kconfig currently controlling compilation of this code is:

config SIMPLE_PM_BUS
        bool "Simple Power-Managed Bus Driver"

...meaning that it currently is not being built as a module by anyone.

In removing the orphaned modular support in a previous patch set,
Geert indicated he'd rather see this code converted to tristate.

I normally don't do that because it extends functionality that I
can't easily run time test or even know if the use case makes sense,
but since in this case the author has nominated it as such, we do
the conversion here.

Note that doesn't change the lack of run time testing ; this change
is only tested for sucessful compile and modpost.

[geert: Ethernet is probed successfully on sh73a0/kzm9g after
        insmodding simple-pm-bus.ko]

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>