PCI: uniphier: remove module code from built-in driver

builtin_platform_driver() and MODULE_* are always odd combination.

This file is not compiled as a module by anyone because
CONFIG_PCIE_UNIPHIER is a bool option.

Let's remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

We explicitly disallow a driver unbind, since that doesn't have a
sensible use case anyway, and it allows us to drop the ".remove" code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>