lib: strto: add simple_strtoll function

Add simple_strtoll function for converting a string containing digits
into a long long int value.

Signed-off-by: Roland Gaudig <roland.gaudig@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/vsprintf.h b/include/vsprintf.h
index 2290083..4016de6 100644
--- a/include/vsprintf.h
+++ b/include/vsprintf.h
@@ -41,6 +41,7 @@
 unsigned long long simple_strtoull(const char *cp, char **endp,
 					unsigned int base);
 long simple_strtol(const char *cp, char **endp, unsigned int base);
+long long simple_strtoll(const char *cp, char **endp, unsigned int base);
 
 /**
  * trailing_strtol() - extract a trailing integer from a string
diff --git a/lib/strto.c b/lib/strto.c
index c00bb58..f8b53d8 100644
--- a/lib/strto.c
+++ b/lib/strto.c
@@ -143,6 +143,14 @@
 	return result;
 }
 
+long long simple_strtoll(const char *cp, char **endp, unsigned int base)
+{
+	if (*cp == '-')
+		return -simple_strtoull(cp + 1, endp, base);
+
+	return simple_strtoull(cp, endp, base);
+}
+
 long trailing_strtoln(const char *str, const char *end)
 {
 	const char *p;