soc: mediatek: SCPSYS: use builtin_platform_driver

SCPSYS can't be built as module. Use builtin_platform_driver instead.
For this probe must not be __init and the data accessed can't be
__initconst. Remove this macros. To make the impact as small as possible,
fold scp_domain_data into scp_domain via a pointer.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>