clk: imx: scu: fix build break when compiled as modules

After commit e0d0d4d8 ("clk: imx8qxp: Support building i.MX8QXP clock
driver as module"), clk-scu.c and clk-imx8qxp.c are complied in one module,
thus there can be only one module_init() in those two files.
Commit 77d8f306 ("clk: imx: scu: add two cells binding support")
introduced another module_init() in clk_scu.c which caused the errors
below.

To fix the issue, we can remove the unnecessary builtin_platform_driver
from clk_scu.c and directly register the driver in imx_clk_scu_init().

  CC [M]  drivers/clk/imx/clk-scu.o
In file included from ../include/linux/of_device.h:6,
                 from ../include/linux/of_platform.h:12,
                 from ../drivers/clk/imx/clk-scu.c:11:
../drivers/clk/imx/clk-scu.c: In function ‘imx_clk_scu_init’:
../drivers/clk/imx/clk-scu.c:176:35: error: ‘imx_clk_scu_driver’ undeclared (first use in this function); did you mean ‘imx_clk_scu_init’?
  176 |  return platform_driver_register(&imx_clk_scu_driver);
      |                                   ^~~~~~~~~~~~~~~~~~
../include/linux/platform_device.h:218:29: note: in definition of macro ‘platform_driver_register’
  218 |  __platform_driver_register(drv, THIS_MODULE)
      |                             ^~~
../drivers/clk/imx/clk-scu.c:176:35: note: each undeclared identifier is reported only once for each function it appears in
  176 |  return platform_driver_register(&imx_clk_scu_driver);
      |                                   ^~~~~~~~~~~~~~~~~~
../include/linux/platform_device.h:218:29: note: in definition of macro ‘platform_driver_register’
  218 |  __platform_driver_register(drv, THIS_MODULE)
      |                             ^~~
../drivers/clk/imx/clk-scu.c:177:1: error: control reaches end of non-void function [-Werror=return-type]
  177 | }
      | ^
At top level:
../drivers/clk/imx/clk-scu.c:470:31: warning: ‘imx_clk_scu_driver’ defined but not used [-Wunused-variable]
  470 | static struct platform_driver imx_clk_scu_driver = {

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 77d8f306 ("clk: imx: scu: add two cells binding support")
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>