platform: Fix compile error caused by standard includes

Avoid using standard includes namely stdint.h, string.h, stdlib.h, etc.

All standard include except stddef.h give compilation error due
to differences in RISC-V toolchain configuration. Typically, the
compilation error is related to "gnu-stubs-lp64.h".

This patch fixes compile error caused by standard includes by
providing substitutes of definetions provided by standard includes
wherever required.

Signed-off-by: Anup Patel <anup.patel@wdc.com>

platform/common/include/plat/string.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2018 Western Digital Corporation or its affiliates.
+ *
+ * Authors:
+ *   Atish Patra <atish.patra@wdc.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#ifndef __STRING_H__
+#define __STRING_H__
+
+#include <sbi/sbi_types.h>
+
+int strcmp(const char *a, const char *b);
+
+size_t strlen(const char *str);
+
+size_t strnlen(const char *str, size_t count);
+
+char *strcpy(char *dest,const char *src);
+
+char *strncpy(char *dest, const char *src, size_t count);
+
+char *strchr(const char *s, int c);
+
+char *strrchr(const char *s, int c);
+
+void *memset(void *s,int c,size_t count);
+
+void *memcpy(void *dest, const void *src, size_t count);
+
+void *memmove(void *dest,const void *src, size_t count);
+
+int memcmp(const void *s1,const void *s2, size_t count);
+
+void *memchr(const void *s, int c, size_t count);
+
+#endif