clk: imx: scu: fix MXC_CLK_SCU module build break

This issue can be reproduced by having a kernel config with
CONFIG_IMX_MBOX=m and CONFIG_MXC_CLK_SCU=m.  It's caused by the Makefile
wanting to build clk-scu.o and clk-imx8qxp.o as different targets but
that doesn't work (e.g. MXC_CLK_SCU = y while CLK_IMX8QXP = n)

"obj-$(CONFIG_MXC_CLK_SCU) += clk-imx-scu.o clk-imx-lpcg-scu.o
clk-imx-scu-$(CONFIG_CLK_IMX8QXP) += clk-scu.o clk-imx8qxp.o"

Having MXC_CLK_SCU=y/m while CLK_IMX8QXP=n will cause a linker problem
like below:

  LD [M]  drivers/clk/imx/clk-imx-scu.o
  arm-poky-linux-gnueabi-ld: no input files

Make MXC_CLK_SCU be un-selectable by users so it can only be selected by
the CLK_IMX8QXP option, ensuring the two symbols are built together.
Drop COMPILE_TEST too because this option isn't selectable anymore. We
can remove it from MXC_CLK_SCU because CLK_IMX8QXP selects MXC_CLK_SCU
which already has COMPILE_TEST.

Fixes: e0d0d4d8

 ("clk: imx8qxp: Support building i.MX8QXP clock driver as module")
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Link: https://lore.kernel.org/r/20201130084624.21113-1-aisheng.dong@nxp.com
[sboyd@kernel.org: Rework commit text]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>