clk: oxnas: make it explicitly non-modular

The Kconfig currently controlling compilation of this code is:

drivers/clk/Kconfig:config COMMON_CLK_OXNAS
drivers/clk/Kconfig:    bool "Clock driver for the OXNAS SoC Family"

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

Lets 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 for non-modular drivers.

Since module_platform_driver() uses the same init level priority as
builtin_platform_driver() the init ordering remains unchanged with
this commit.

Also note that MODULE_DEVICE_TABLE is a no-op for non-modular code.

Cc: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160704211220.5685-6-paul.gortmaker@windriver.com