drivers/soc: make mediatek/mtk-scpsys.c explicitly non-modular

The Kconfig currently controlling compilation of this code is:

drivers/soc/mediatek/Kconfig:config MTK_SCPSYS
drivers/soc/mediatek/Kconfig:   bool "MediaTek SCPSYS Support"

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

Lets remove the couple traces of modularity so that when reading the
driver there is no doubt it is builtin-only.

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

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Daniel Kurtz <djkurtz@chromium.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>