Add mbed_lib.json for PSOC6 SCL to exclude it from build metal build

SCL is for the Cypress Ethernet driver which is not currently
compatible with bare metal.

Author: LDong-Arm

targets/TARGET_Cypress/TARGET_PSOC6/COMPONENT_SCL/mbed_lib.json

@@ -0,0 +1,3 @@
+{
+  "name": "cy_psoc6_scl"
+}