pk: move ECC setters to a separate file

- These functions are used both in pkparse.c for key parsing
  as well as pk.c for managing copy_from_psa(). As as consequence
  they should belong to pk.c, but that would make that module
  messier, so that's why a new separate module is added.
- Their guard can be changed from PKPARSE_C to PK_C.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 47ecf17..835604f 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -56,6 +56,7 @@
     padlock.c
     pem.c
     pk.c
+    pk_ecc.c
     pk_wrap.c
     pkcs12.c
     pkcs5.c