lib: add asn1 decoder

Imported from linux kernel v5.3:
 lib/asn1_decoder.c with changes marked as __UBOOT__

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index 499470c..1d1debc 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -569,6 +569,11 @@
 config ASN1_COMPILER
 	bool
 
+config ASN1_DECODER
+	bool
+	help
+	  Enable asn1 decoder library.
+
 source lib/efi/Kconfig
 source lib/efi_loader/Kconfig
 source lib/optee/Kconfig