psa: Add initializers for key policies

Add new initializers for key policies and use them in our docs, example
programs, tests, and library code. Prefer using the macro initializers
due to their straightforwardness.
diff --git a/tests/suites/test_suite_psa_crypto.data b/tests/suites/test_suite_psa_crypto.data
index 848e8ed..09029ff 100644
--- a/tests/suites/test_suite_psa_crypto.data
+++ b/tests/suites/test_suite_psa_crypto.data
@@ -332,6 +332,9 @@
 PSA key policy set and get
 key_policy:PSA_KEY_USAGE_ENCRYPT:PSA_ALG_CBC_NO_PADDING
 
+Key policy initializers zero properly
+key_policy_init:
+
 PSA key policy: MAC, sign | verify
 depends_on:MBEDTLS_MD_C:MBEDTLS_SHA256_C
 mac_key_policy:PSA_KEY_USAGE_SIGN | PSA_KEY_USAGE_VERIFY:PSA_ALG_HMAC(PSA_ALG_SHA_256):PSA_KEY_TYPE_HMAC:"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa":PSA_ALG_HMAC(PSA_ALG_SHA_256)