clk: scmi: Add Kconfig option for SPL

Building U-Boot SPL with CLK_SCMI and SCMI_FIRMWARE Kconfig options
enabled and SPL_FIRMWARE disabled result in the following error.

  drivers/clk/clk_scmi.o: in function `scmi_clk_gate':
  drivers/clk/clk_scmi.c:84: undefined reference to `devm_scmi_process_msg'
  drivers/clk/clk_scmi.c:88: undefined reference to `scmi_to_linux_errno'
  drivers/clk/clk_scmi.o: in function `scmi_clk_get_rate':
  drivers/clk/clk_scmi.c:113: undefined reference to `devm_scmi_process_msg'
  drivers/clk/clk_scmi.c:117: undefined reference to `scmi_to_linux_errno'
  drivers/clk/clk_scmi.o: in function `scmi_clk_set_rate':
  drivers/clk/clk_scmi.c:139: undefined reference to `devm_scmi_process_msg'
  drivers/clk/clk_scmi.c:143: undefined reference to `scmi_to_linux_errno'
  drivers/clk/clk_scmi.o: in function `scmi_clk_probe':
  drivers/clk/clk_scmi.c:157: undefined reference to `devm_scmi_of_get_channel'
  make[1]: *** [scripts/Makefile.spl:527: spl/u-boot-spl] Error 1
  make: *** [Makefile:2043: spl/u-boot-spl] Error 2

Add Kconfig option so that CLK_SCMI can be disabled in SPL to fix this.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>