drivers/platform: Convert non-modular pdev_bus to use builtin_platform_driver

This driver is configured with a Kconfig option that is
declared as a bool.  Hence it is not possible for the code
to be built as modular.  However the code is currently using
the module_platform_driver() macro for driver registration.

While this currently works, we really don't want to be including
the module.h header in non-modular code, which we'll be forced
to do, pending some upcoming code relocation from init.h into
module.h.  So we fix it now by using the non-modular equivalent.
And since we've already established that the code is non-modular,
we can completely drop any code relating to module_exit.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>