soc: rockchip: fixed compilation error

This adds fixed below errors when compiled rockchip_defconfig without
CONFIG_RK3368_SCPI_PROTOCOL select:

In file included from drivers/clk/rockchip/clk-ddr.c:23:0:
include/soc/rockchip/scpi.h:89:12: warning:
'scpi_sys_set_jtagmux_on_off' defined but not used [-Wunused-function]
error, forbidden warning: scpi.h:89
scripts/Makefile.build:258: recipe for target
'drivers/clk/rockchip/clk-ddr.o' failed
make[3]: *** [drivers/clk/rockchip/clk-ddr.o] Error 1

Change-Id: I5abc184554dcfc3697be82aede8dec27da2fcdd9
Signed-off-by: Frank Wang <frank.wang@rock-chips.com>