rockchip: rk3399: Add missing sentinel in syscon

when enable PMIC rk808,the system will halt at very
 early stage,log is shown as bellow.

INFO:    plat_rockchip_pmu_init(1211): pd status 3e
INFO:    BL31: Initializing runtime services
INFO:    BL31: Preparing for EL3 exit to normal world
INFO:    Entry point address = 0x200000
INFO:    SPSR = 0x3c9
time 44561b, 0 (<<----Just stop here)

It's caused by the absence of "{ }" in syscon_rk3399.c
,which will lead to memory overflow like below.According
 to Sysmap file ,we can find the function buck_get_value
of rk808 is just follow the compatible struct,the pointer
"of_match" point to "buck_get_value",but it is not a
struct and don't have member of compatible, In this case,
system crash. So,on the face, it looks like that rk808 is
guilty.but he is really innocent.

while (of_match->compatible) { <<----------
    if (!strcmp(of_match->compatible, compat)) {
    *of_idp = of_match;
    return 0;
    }
    of_match++;
}
Signed-off-by: NEric Gao <eric.gao@rock-chips.com>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>
Tested-by: NKever Yang <kever.yang@rock-chips.com>

arch/arm/mach-rockchip/rk3399/syscon_rk3399.c

@@ -14,6 +14,7 @@ static const struct udevice_id rk3399_syscon_ids[] = {
 	{ .compatible = "rockchip,rk3399-pmugrf", .data = ROCKCHIP_SYSCON_PMUGRF },
 	{ .compatible = "rockchip,rk3399-pmusgrf", .data = ROCKCHIP_SYSCON_PMUSGRF },
 	{ .compatible = "rockchip,rk3399-cic", .data = ROCKCHIP_SYSCON_CIC },
+	{ }
 };
 
 U_BOOT_DRIVER(syscon_rk3399) = {