clk: scmi: claim the dependency on CONFIG_CLK

Without CONFIG_CLK, the build fails with the following message:
  LD      u-boot
aarch64-none-linux-gnu-ld.bfd: drivers/firmware/scmi/scmi_agent-uclass.o: \
			in function `scmi_bind_protocols':
.../drivers/firmware/scmi/scmi_agent-uclass.c:79: undefined reference to \
			`_u_boot_list_2_driver_2_scmi_clock'

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>