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/Makefile b/lib/Makefile
index 90d4425..f69c013 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -17,6 +17,7 @@
 obj-$(CONFIG_CMD_DHRYSTONE) += dhry/
 obj-$(CONFIG_ARCH_AT91) += at91/
 obj-$(CONFIG_OPTEE) += optee/
+obj-$(CONFIG_ASN1_DECODER) += asn1_decoder.o
 
 obj-$(CONFIG_AES) += aes.o